We study two possible mechanisms of sulfur (S) poisoning of copper (Cu) surfaces: S adsorption and formation of Cu-S compounds. Based on the performed calculations, we predict new Cu-S phase diagram that not only describes the formation of Cu-S compounds but also predicts a dependence of the transition temperature between clean and contaminated Cu surfaces on partial pressure ratio of H-2 and H2S (P-H2/P-H2S). Since conditions under which sulfur sorbs and desorbs at Cu surfaces cannot be predicted directly from the classical thermodynamic database, our study enhances current understanding of Cu-S interaction and mechanisms of S poisoning of Cu-based catalysts. (C) 2012 Elsevier B. V. All rights reserved.