The structural transformations induced by heating on MoO3 mesoporous samples grown in vapor-phase under helium and hydrogen were studied by in situ Raman spectroscopy. The samples were continuously irradiated by a He-Ne laser of 5.5 mW for 105 min and Raman spectra were dynamically acquired every 5 min, in order to evaluate the laser effects. The He-grown sample did not undergo structural transformations due to the laser irradiation while the H-2-grown sample underwent changes after just 10 min of irradiation. On the other hand, each type of sample was heated in air from room temperature up to 450 degrees C using a heating rate of 5 degrees C min(-1) and Raman spectra were recorded each 25 degrees C. The He-grown sample remained structural unchanged up to 250 degrees C revealing a high temperature state of its amorphous matrix whereas the H-2-grown sample exhibited changes around 70 degrees C. These changes were attributed to the crystallization onset of its amorphous matrix to beta-MoO3. However, this same transition was detected between 250 degrees C and 275 degrees C for the He-grown sample. A second transformation was detected in both samples in a temperature range more similar associated to the transition of the formed beta-MoO3 phase to alpha-MoO3. After the heat treatment, the oxide of both samples was slightly reduced, as revealed by XPS analysis. The metastability at low temperatures of the amorphous phase of the H-2-grown sample could be associated to release of hydrogen trapped inside the compound and to the break of weak bonds between Mo and OH groups. At high temperatures, above 225 degrees C, the rapid conversion to beta-MoO3 of the amorphous matrix of the H-2-grown sample occurred due to the dehydration of the sample. (C) 2012 Elsevier B.V. All rights reserved.