Crystalline VO2.5 films were prepared by reactive dc magnetron sputtering followed by annealing posttreatment. Lithium was inserted electrochemically from an electrolyte so that LixVO2.5 (0<x<1.5) was formed. The evolution of the electromotive force (EMF) was recorded during Li intercalation. For lithiated samples, structure by x-ray diffraction, optical properties by spectrophotometry in the 0.3-2.5 μm range, infrared reflectance by spectrophotometry in the 400-1200 cm-1 interval, and the mechanical stress by beam deflectometry were studied. Changes in lattice parameters, phonon spectra, and stress levels gave a consistent picture of the structural evolution. Measurements of optical absorption and EMF were interpreted within a conceptual model of interband transitions between the O 2p band and a split V 3d band. Durability and reversibility of the Li intercalation/deintercalation were verified by potentiodynamic measurements. For comparison, the properties of highly disordered films were also measured. © 1995 American Institute of Physics.