W.H. Aschbacher C.-A. Pillet Non-Equilibrium Steady States of the XY Chain (228K, postscript) ABSTRACT. We study the non-equilibrium statistical mechanics of the two-sided XY chain. We start from an initial state in which the left and right part of the lattice {x|x<-M} and {x|x>M}, are at inverse temperatures beta_L and beta_R. Using a simple scattering theoretic analysis, we construct the unique non-equilibrium steady state (NESS). This state depends on beta_L and beta_R, but not on the choice of the decoupling parameter M. We prove that in the non-equilibrium case, beta_L \not= beta_R, this state has strictly positive entropy production.