To enhance the electrochromism of vanadium oxide films used as a counter electrode of tungsten oxide (WO3) films in electrochromic (EC) devices, V-M (Pr, Nd, Sm, Dy) oxide films were synthesized by vacuum evaporation and annealed at 400 degreesC. The effects of 25% rare earth addition on electrochromism, structure and optical properties of the vanadium oxide films were investigated. By using cyclic voltammetry, it was found that all V-M (Pr, Nd, Sm, Dy) oxide films heated at 400 degreesC for 30 min showed ten times better cycling stabilities in PC solution than vanadium oxide films. In situ UV-vis-NIR spectroelectrochemical measurements confirmed that all V-M oxide films had higher transmittances than vanadium oxide film under the colored state in vis and NIR. Of the V-M oxide films, V-Sm oxide film showed a very small coloration efficiency (CE; 0.6) in vis and NIR, and showed good ionic and electronic conduction, implying its potential application as a counter electrode in EC devices. From XRD results, the formation of orthovanadate SmVO4 and DyVO4 was testified in V-Sm and V-Dy oxide films, respectively. Moreover, a close relationship between the doping effects of M3+ and its radius was found. (C) 2002 Elsevier Science B.V. All rights reserved.