He had allowed several people from Eastern Europe to stay with him in the Netherlands as they escaped war and persecution, including Polish Jews and Russian dissidents of communism.

Known for his "unbridled energy and pig-headedness" in teaching this way – for which he was reprimanded by the scholastic board – he remained an important influence, correspondent, and confidant after van Wijk's graduation.

"[14] Van Wijk had made an impression with another professor, C. C. Uhlenbeck, then employed by the university as a buitengewoon hoogleraar [nl] though the following year he was promoted to a full professorship.

[22] After passing his doctoraal exam – granting him the equivalent of a master's degree – in May 1901, van Wijk returned to Zwolle on 7 October to begin writing his doctoral dissertation.

On 4 June 1905, Johan Tjeenk Willink – a former schoolmate of van Wijk's at gymnasium – wrote to inquire about the latter writing a Dutch grammar textbook for young students.

[52] He moved to The Hague on 26 January and began working under W. G. C. Byvanck [nl], who was controversial for his unorthodox teaching style and abrasive personality, but he and van Wijk enjoyed a warm affection for each other.

[54] Roughly six months after his appointment to the Royal Library, he was granted a month-long leave of absence to travel to Russia by Ministry of Internal Affairs with the support of Byvanck.

[60] He published his frustration in the academic journal De Nieuwe Taalgids [Wikidata] ('The New Language Guides'), writing: "Is it not then shameful that we still know so little of the history and mutual relations of our dialects?"

[68] Van Wijk's editors for these publications often criticized his writing style, but his expertise was unique, he made no factual errors, and he was able to maintain a regular reading audience.

[72][73] In 1911, Abraham Kuyper publicly supported Croiset van der Kop as the obvious choice for the chairmanship in De Standaard, citing that she came "emphatically recommended [...] by Russian scholars" and later urged the Second Chamber of the Dutch parliament to create the chair.

[91] From Prague, van Wijk traveled on to Warsaw, then a part of Russian-occupied Poland, spending three days there and collaborating with several locals in search of books, most notably the Belarusian linguist Yefim Karsky.

[92] Following Warsaw, van Wijk made landfall in Saint Petersburg, where he was received by the Dutch envoy, Arthur Martin Désiré baron Sweerts de Landas Wyborgh.

[102] Van Wijk then traveled to Agram (modern-day Zagreb, Croatia), then a part of the Austro-Hungarian Empire, where he was similarly confronted with absentee intellectuals he had intended to meet.

[102] He met briefly with Franjo Fancev [hr] to discuss the Kajkavian dialect which was native to the area in and around Agram, before continuing on to Laibach (modern-day Ljubljana, Slovenia) for a day.

[103] Van Wijk met with Fran Ilešič [sl], chairman of the Slovenian equivalent of Matica Hrvatska, and Oton Župančič before entering Austria proper following the assassination of Archduke Franz Ferdinand, though he makes no mention of the event in his report and appears to have been completely ignorant that it happened.

When word finally reached him about Franz Ferdinand's assassination, he wrote that he had found the political conversations with the local Viennese Croats and Slovenes informed him greatly.

[107] Van Wijk ended his trip with several weeks in Zakopane in the Tatra Mountains of Poland with Wiktor Porzeziński [ru; bg], a linguist at University of Moscow.

[108] He wrote: "And if I succeed in getting people to understand that; in other words, if I can make the reader see how a journey of few months, staying in many cities only a few days, is nevertheless sufficient to produce such fantastic results for our national scholarship, then I have achieved all I could wish.

In van Wijk's first ever piece for a newspaper, writing for the Nieuwe Rotterdamsche Courant on 15 September, he wrote a retrospective about his time traveling to Poland, providing a glimpse into the human toll of the war: A year ago, Glów was a prosperous village with fine farmhouses built in stone below and wooden upper floor of the type that is tytpical of this region, but today Glów stands no highter than 10 centimetres [4 in] above the ground: the foundations of houses, nothing more.

[...] Nothing has made a more sinister impression on me than the deadly silence in this village that had been wiped from the face of the earth, where a few hungry souls clad in rags remained to kiss our hand meekly (as is the habit in these parts) and tell us that they had nothing anymore.

[112] At one point, the Catholic Archbishop of Lviv, Józef Bilczewski, requested van Wijk's assistance in getting twenty wagonloads of rice that were being held in Rotterdam released to Poland.

[124] Leonid suffered from some kind of respiratory disease; van Wijk regularly visited him in Czechoslovakia, where the air was purportedly healthier for his condition, and when he was hospitalized in Leiden.

[128] This did not appear to sway the government's opinion; on 14 November, Leiden's police commissioner wrote about the concern to the attorney general of the court of justice in The Hague: Although there was never proof to be had in Leiden that Professor VAN WIJK has held communist sympathies, it was frequently drawn to our attention that the said professor on any occasions received, assisted, and even frequently accommodated for considerable periods, unkempt individuals of the East European population.

[131] Van Wijk, unlike his mentor Uhlenbeck, usually shied away from fieldwork, but during the winter of 1930 and 1931, he was shown two sheets in Serbian Old Church Slavonic on the Desert Fathers which piqued his interest.

His work led him to stay at a monastery in Dalmatia for two weeks in August 1934 to study a manuscript kept there; the resulting publication was van Wijk's only contribution in the field written in Serbo-Croatian.

[132] At some point during the late 1930s, van Wijk wrote and completed a critical manuscript on the Skete Patericon, an Old Church Slavonic translation of the Ancient Greek text Andrō̂n hagíōn bíblos (Ἀνδρῶν ἁγίων βίβλος lit.

[140] Although the general atmosphere was bleak, his former student F. B. J. Kuiper reports that van Wijk remained "basically optimistic", believing that the social order dominated by national borders would become less prominent in the post-war era.

[142] On 22 March 1941, van Wijk fell ill after a dinner with Eduard Meijers, a Jewish law professor whose dismissal caused the university to be shut down in protest.

[161][160] During his lifetime, van Wijk published over 600 works covering a broad spectrum of topics; about fifty of these focused on Slavic literature, with all but three intended for a Dutch-speaking audience.

[167] Despite his strong opposition to the Bolsheviks, van Wijk remained friendly with those who were sympathetic to the movement, most notably his students Jan Romein, Annie Romein-Verschoor, and Francis de Graaff.