The effect of ion size and valency on the optical and structural properties of polycrystalline MoO3 films has been investigated by recording these properties before and after intercalating the films with H, K, and Mg ions of different concentrations. X-ray diffraction (XRD) measurements on intercalated films show lattice distortions/structural disorder depending on the size of intercalant ion. Spectral variation of transmittance (T) and absorptance (100-R-T) was found to depend on the concentration of intercalated ions (x), their size, and valency. Absorption in intercalated films in the visible and near-infrared region is due to lower valence states of Mo. Optical and XRD studies show superiority of Mg ions over other monovalent ions as intercalants. (c) 2005 The Electrochemical Society.