The role of the step sites in the water gas shift reaction catalyzed by Cu surfaces has been studied by using the Cu(321)-stepped surface as a representative model and periodic density functional theory within a supercell approach. Several reaction pathways were considered and the corresponding transition states for the elementary steps on each pathway were located and characterized. It was found that the presence of steps favors the associative route through the carboxyl intermediate assisted by co-adsorbed OH. The presence of step sites decreases the activation energy barriers for the rate-limiting steps, compared to the perfect Cu(111) surface. Reaction rate constants for the different pathways involved in the two molecular mechanisms, obtained from transition state theory, are reported. Finally comparison to previous work allows one to propose a useful Bronsted-Evans-Polanyi relationship. (C) 2009 Elsevier Inc. All rights reserved.