Bismuth oxide thin films prepared by sputtering or vacuum evaporation were found to be a new electrochromic material. The best electrochromic performance in propylene carbonate containing 1 M LiClO4 was observed with the sputtered film annealed at 300-400 degrees C in air for 30 min. The film showed a clear electrochromic change between transparent and dark brown on switching between anodic (+1.2 V) and cathodic (-2.0 V) polarization. Coloration and decoloration were completed within about 10 s with a coloration efficiency of 3.7 cm(2) . C-1.