His doctoral research explored the phenomenon of superconductivity at the University of Leiden, following the lead of Heike Kamerlingh Onnes and Wander Johannes de Haas.

He completed his PhD in 1926, with a thesis entitled: Onderzoekingen over den suprageleidenden toestand van metalen,[1] under Wander Johannes de Haas.

In as early as 1929, it was decided that Sizoo act as professor at the newly established Faculty of Mathematics and Physics.

Sizoo promoted the then emerging field of radioactivity, partly because there were no other laboratories in the Netherlands exploring this area depth.

In 1943 the Vrije Universiteit closed its doors, and scientific work in the ensuing time was much more difficult, despite efforts by Sizoo to keep the laboratory equipment from the hands of German occupiers.