Atomic force microscopy has been used to characterize the evolution of MoO3(010) growth sand cleavage surfaces during reactions with air-N-2-MeOH mixtures between 200 and 300 degrees C. At reaction temperatures greater than or equal to 250 degrees C, closed step loops, bounded by undercoordinated surface Mo sites, nucleate and grow as the (010) surface volatilizes in a nearly layer-by-layer fashion. The density and crystallography of the new surface sites depend on the composition of the gas mixture in the reactor. In air-rich environments, the step loops have a rectangular shape and are elongated along [001]. At any instant, only the uppermost one or two (010) layers are affected by the process. As the concentration of Oz in the feed is reduced, the shape evolves to elongated triangles oriented along [100] and the loops exist on multiple layers simultaneously. During reactions, the observed linear step edge densities can increase from 1x10(3)/cm to greater than 1.5 x 10(5)/cm. The potential influence of these undercoordinated surface Mo sites on the reactivity of particles with different aspect ratios is discussed.