In 1927, Kronig returned to Europe for good and worked in different prominent centres of research: Copenhagen, London, Zürich (where for a year he was Pauli's assistant). Around 1930 he settled in the Netherlands: first in Utrecht, then in Groningen, first as Dirk Coster's assistant, and from 1931 as an associate professor, and since 1939 as a full professor at the Delft University of Technology where he stayed until his retirement in 1969. Between 1959 and 1962 he was the rector of the university. He was recognized internationally by then as a renowned theorist who corresponded with the leading characters of that time and made interesting contributions to quantum mechanics and the application of it particularly on the physics of molecules and molecular spectra, an area on which he was the expert of those days.