Post-growth rapid thermal annealing of alpha-MoO3 layered crystals in Pb-species that were in situ evaporated from PbO coatings has been studied using various techniques. They reveal the occurrence of solid-gas reactions between MoO3 and PbO, which is accompanied by MoO3 sublimations, leading to remarkable morphology and structure evolutions when the annealing temperature (T-ann) is increased from 400 to 800 degrees C. Onset of nucleation and crystal growth of PbMoO4, which initiated at the structural defects of alpha-MoO3, occurred at 600 degrees C. The growth of PbMoO4 and the consumption of MoO3 at their boundary areas give rise to materials melting over there at 700 degrees C, which, in turn, connected the localized grown PbMoO4 via capillary actions. After cooling down to room temperature, the PbMoO4 solidified in crystals, while the MoO3 consisted of both amorphous and crystalline precipitates with regular in-plane orientations. In comparison, spherical domes that appeared at T-ann = 800 degrees C resulted from enhanced localized reactions between the molten PbMoO4/MoO3 and the Al2O3 substrate. These results shed new lights on thermal behavior of alpha-MoO3 and the temperature-dependent reactions between MoO3 and PbO.