Résumé : In this paper we express the dressed-particle theory of Prigogine and coworkers in terms of reduced distribution functions. The reduced dressed-particle distribution functions are factorized (statistical independence of dressed particles) at thermal equilibrium if a formal condition (the so-called "equilibrium condition") is satisfied. The dressed one-particle distribution function is then sufficient to characterize all thermodynamic quantities; in particular, the entropy takes the same form as that of a perfect gas. We discuss our results and indicate how to generalize them out of equilibrium. © 1972.