In his first years at Leiden University, Mazur studied the classical and quantum molecular foundations of non-equilibrium thermodynamics. Significant results included the derivation of the Langevin equation with Irwin Oppenheim and the classic paper on harmonic oscillator systems by George Ford, Mark Kac, and Mazur, which was published in the Journal of Mathematical Physics (in 1965). Mazur's work in the 1950s and 1960s culminated in the publication of Nonequilibrium Thermodynamics (North Holland and Interscience, 1962), written by de Groot and Mazur. This book, translated into several languages, became a classic in the field and was later republished as a series of classic monographs.