Atomic force microscopy was used to characterize the nanometer-scale structural evolution of the MoO3 (010) surface during reaction with hydrogen at 400 degrees C. Two primary surface modifications were identified. First, water vapor, when present in the reactor either as an impurity or as an oxidation product, accelerates the volatilization of MoO3 and leads to the formation of surface voids. The presence of these voids increases the density of (100) and (h01)-type surface sites on the basal planes. Second, oxygen removal leads to the formation of crystallographic shear planes. The intersection of these defects with the (010) surface creates 2 Angstrom high steps along [001] and their presence indicates that the surface vacancy concentration has reached an upper limit. The mechanisms by which these structural changes might influence the reactivity of molybdenum oxide catalysts are discussed.