There, under the tutelage of professors van Lennep and Wilmett he, though far from abandoning classical languages (especially ancient and byzantine Greek), focused on oriental studies.
From 1817–1822 he held the post of "extraordinary" professor Oriental languages and title of Interpres Legati Warneriani (Interpreter of the legacy of Levinus Warner) at Leiden University.
In 1820 Hamaker published his Specimen of a catalog of oriental manuscripts of Warner's bequest, which included descriptions of the Futuh (Conquests) of Baladhuri, a section of Al-Tabari's great history, the Murudj al-Dhahab (Fields of Gold) of Al-Masudi, etc.
Hamaker was the first Dutch scholar to give a series of eight public lectures (in 1834; published 1835) on the comparison of Greek, Latin and the Germanic languages with Sanskrit, thus instigating (though not establishing) the study of Indo-European comparative linguistics in Holland.
Among his treatises, works and numerous memoirs included in various collections are the following: Great Universal Dictionary of the Nineteenth Century Media related to Hendrik Arent Hamaker at Wikimedia Commons