The energy profile for the water-gas-shift reaction has been calculated on the active sites of the industrially used Cu/ZnO/Al2O3 catalyst using the BEEF-vdW functional. Our theoretical results suggest that both active site motifs, a copper (211) step as well as a zinc decorated step, are equally active for the water-gas-shift reaction. We find that the splitting of water into surface OH* and H* constitutes the rate-limiting step and that the reaction proceeds through the carboxyl mechanism. Our findings also suggest that mixed copper-zinc step sites are most likely to exhibit superior activity.