Jan Derezinski and Vojkan Jaksic Return to Equilibrium for Pauli-Fierz Systems (626K, postscript) ABSTRACT. We study ergodic properties of Pauli-Fierz systems---$W^*$-dynamical systems often used to describe the interaction of a small quantum system with a bosonic free field at temperature $T\geq0$. We prove that, for a small coupling constant uniform as the positive temperature $T\downarrow 0$, a large class of Pauli-Fierz systems has the property of return to equilibrium. Most of our arguments are general and deal with mathematical theory of Pauli-Fierz systems for an arbitrary density of bosonic field.