The initial stage of spreading of micron-sized MoO3 crystals deposited on a flat Al2O3 support have been studied by synchrotron radiation spectromicroscopy. The lateral resolution of 150 nm and the surface sensitivity of the microscope allowed us to detect submonolayer quantities of Mo-oxide, spread on the support in the vicinity of the MoO3 crystals after annealing in oxygen atmosphere. Detailed analyses of the Mo-oxide wetting the support have been performed combining chemical imaging with spectroscopy. The concentration of the spread Mo-oxide was quantified as a function of the distance from the MoO3 crystals. The determined concentration profiles were fitted to the analytical solution of an equivalent diffusion model for the MoO3 mass transport in dry oxygen at 630 K yielding a diffusion constant of D = 7.8 x 10(-11) cm(2)/s. The results are discussed in the framework of the possible mass transport mechanisms causing the spreading. The simple unrolling carpet was ruled out as an active process. (C) 2000 American Institute of Physics. [S0021-9606(00)70111-5].