MoO3 thin films were intercalated with Mg ions using a standard three-electrode configuration. The concentration of Mg ions was varied from x = 0.05 to x = 0.4,. where x is the ratio of Mg ions and host Mo ions. IR studies were carried out on unintercalated, intercalated, deintercalated and aged films. The effect of ion concentration (x) on lattice distortion and reversibility has been investigated by monitoring systematically the number of resonance absorption peaks, their respective intensities and spectral locations. At low values of x (<0.2) the lattice expansion along b-axis was observed and beyond which lattice distortion alona a-axis was seen. The presence of characteristic resonance absorption peaks at 988, 808 and 537 cm(-1) confirms formation of layered orthorhombic MoO3 phase. IR studies of deintercalated films confirm that the chemical changes occurring at terminal double bonds are irreversible while these are reversible at other sites. The stability of intercalated films was investigated by monitoring the IR reflectance time to time after exposing them to ambience. No measurable degradation was observed even after 6 months. (C) 2004 Elsevier B.V. All rights reserved.