They had two daughters and two sons: Tatyana ('Tanja') (1905–1984) also became a mathematician; Galinka ('Galja') (1910–1979) became an author and illustrator of children's books; Paul, Jr. ('Pavlik') (1915–1939) became a physicist; and Vassily ('Wassik') (1918–1933).

Felix Klein, dean of the Göttingen mathematicians and chief editor of the Enzyklopädie der mathematischen Wissenschaften ("Encyclopedia of Mathematical Sciences"), had counted on Boltzmann for a review about statistical mechanics.

It is a review of the work of Boltzmann and his school, and has a style all its own: a sharp logical analysis of the fundamental hypotheses, clear delineation of unsolved questions, and an explanation of general principles by cleverly chosen transparent examples.

Moreover, because he was unwilling to declare belief in any religious denomination, he could not apply for a professorship and therefore had no prospect of securing a permanent position.

To stimulate interaction and exchange among physics students, Ehrenfest organized a discussion group and a study association called De Leidsche Flesch ("The Leyden jar").

In his lectures, Ehrenfest emphasized simple models and examples to illustrate and clarify basic assumptions.

Among his students were Johannes Burgers, Hendrik Kramers, Dirk Coster, George Uhlenbeck and Samuel Goudsmit, who became famous for jointly proposing the concept of electron spin, Jan Tinbergen, Arend Rutgers, Hendrik Casimir, Gerhard Dieke, Dirk Struik, and Gerard Kuiper.

His assistants included Yuri Krutkov, Viktor Trkal, Adriaan Fokker, Paul Epstein, and Gregory Breit.

Other young foreign scientists who spent extended periods in his laboratory included Gunnar Nordström, Enrico Fermi, Igor Tamm, Oskar Klein, J. Robert Oppenheimer, Walter Elsasser, Ralph Kronig, Werner Heisenberg, Paul Dirac, and David Dennison.

By August 1932, Einstein was so worried that he wrote to the Board of the University of Leiden, expressing concern and suggesting ways in which Ehrenfest's workload could be reduced.

Having made arrangements for the care of his other children, on 25 September 1933, in Amsterdam, Ehrenfest fatally shot his younger son Wassik, who had Down syndrome, then killed himself.

But if, on the contrary, you want to spend at least your first few months patiently, comfortably, and joyfully in discussions that keep coming back to the same few points, chatting about a few basic questions with me and our young people—and without thinking much about publishing (!!!

After Niels Bohr's first visit to Leiden in 1919, for Hans Kramers' thesis defense, he wrote to Ehrenfest: I am sitting and thinking of all what you have told me about so very many different things, and whatever I think of I feel that I have learned so much from you which will be of great importance for me; but, at the same time, I wish so much to express my feeling of happiness over your friendship and of thankfulness for the confidence and sympathy you have shown me, I find myself so utterly incapable of finding words for it.

On the occasion of the 50th anniversary of Lorentz' doctorate (December 1925) Ehrenfest invited both Bohr and Einstein over to Leiden, in an attempt to reconcile their scientific differences about the emerging quantum theory.

These discussions were continued at the 1927 Solvay Conference, where Ehrenfest much to his dismay preferred Bohr's argument in this great debate.

Ehrenfest used colourful German language in his physics lectures:[20] Das ist der springende Punkt (translation: That is the crucial point) Das ist wo der Frosch ins Wasser springt (That is where the frog jumps into the water) Das ist der Patentanspruch (That is the patent claim, the essence) Da hat Herr ... schliesslich die Ratte aus der Suppe gezogen (There Mr. X finally pulled the rat out of the soup) [ when a scientist had solved a messy problem] Je besser man's versteht um so besser steht es dort (The better one understands, the better it is written there, [Ehrenfest's comment when Dirac was asked in writing for an explanation of his work, and Dirac characteristically simply reproduced exactly his previous explanation.]