The spreading of micron-sized MoO3 particles deposited on model supports, alumina and titania thin films, has been studied by scanning photoemission spectromicroscopy. It was shown that Mo species released from the MoO3 crystals spread over the thin oxide film under conditions (heating to 720 K for 6 h in air) similar to those used for catalysts on powdered supports. The changes in the surface morphology were followed by elemental mapping and photoelectron spectroscopy from submicron spots where the intensities and kinetic energies of the Mo 3d, Al 2p, Ti 3d, and O 1s photoelectrons were used as fingerprints for the chemical composition of the surface. The high spatial resolution when imaging the spread phase on the laterally inhomogeneous samples made it possible to throw light onto the active transport mechanism of the wetting Mo oxide species during heat treatment of the model systems.