The spreading of MoO3 crystallites on a flat Al2O3 support has been investigated by means of integrative X-ray photoelectron spectroscopy and laterally resolving photoelectron spectromicroscopy. The effect of different heat treatments on the spreading of the MoO3 crystallites has been studied by systematically varying the temperature, the total pressure, and the humidity of the oxygen atmosphere. The chemical images and the quantitative analysis of the lateral expansion of the spread phase around a single MoO3 crystallite provided insight in the effect of the parameter changes, since the same crystallite could be monitored and the spread layer was removed prior to each heat treatment. The obtained data set strongly supports that gas-phase transport is responsible for the spreading and wetting of molybdate species on the support surface. A model is discussed that consistently explains all experimental observations.