A microkinetic model for the kinetics of the water-gas shift over sulfided CoMo/Al2O3 catalysts was developed starting from a similar model for unpromoted sulfided Mo/Al2O3 catalysts. Co was found to promote the catalyst’s activity only at low CO/H2O ratios at high ratios the Mo catalyst was marginally more active than the CoMo catalyst. The most important difference between the two models was the strength of interactions between the surface and hydroxyl groups. The addition of Co increased the stability of hydroxyl groups relative to sulfhydryl groups, and at higher H2O concentrations this allowed oxidized surface sites to more readily participate in both steam adsorption and hydrogen desorption steps. The results are most easily reconciled in terms of a promotional model where the Co and Mo are in close proximity and the active sites are similar to sites on unpromoted Mo catalysts.