The effect of microstructure and stoichiometry of MoO3 films on hydrolysis has been thoroughly investigated and reported in this Letter. MoO3 films having different cluster size, stoichiometry and crystal structure were prepared and deliberately exposed to identical high humidity conditions for different durations. Infrared spectroscopy was used to monitor hydroxylation process in these films. Polycrystalline films with large grain size were found to be inert to humidity conditions establishing the role of structure in hydroxylation process. All amorphous films adsorb water, part of which is dissociated to form Mo-OH, Mo-O-H bonds giving rise to two closely placed absorption peaks in 2500-4000 cm(-1) spectral range. The concentration of these bonds and free water are strongly influenced by film stoichiometry and microstructure. (c) 2005 Elsevier B.V. All rights reserved.