When he returned to the Netherlands he started a combined study of physics, chemistry and maths at the University of Amsterdam.
With the German invasion of the Netherlands in May 1940, Van der Waals was called for military service for the mounted artillery.
[4][1] After the war ended, Van der Waals finished his studies at the University of Amsterdam in October 1945.
[1] He obtained his doctorate at the University of Groningen in 1950, with a thesis titled Thermodynamic Properties of Mixtures of Alkanes Differing in Chain Length.
[1][4] Van der Waals was appointed professor of experimental physics at Leiden University in 1967, and retired in 1989.
[5] Van der Waals was elected a member of the Royal Netherlands Academy of Arts and Sciences (KNAW) in 1971.
[9] Van der Waals has been involved in the conservation and restoration of the Trippenhuis, the seat of the KNAW, since the 1980s.