The effects of tantalum oxide (Ta2O5) films deposited on tungsten oxide (WO3) films were investigated since unmodified WO3 films have a tendency to be soluble in the acidic electrolyte. The electrochromic properties of the resulting Ta2O5/WO3 films were then compared to those of unmodified WO3 films by means of in situ transmittance measurements. In the case of WO3 films, electrochromic coloration disappeared as the potential cycling proceeded. On the other hand, Ta2O5/WO3 films showed a stable coloration over the entire potential cycling procedure and had a good memory effect under open-circuit voltages. This indicates that Ta2O5 films play a desired role as a protective layer for the WO3, inhibiting its dissolution in the acidic electrolyte. (C) 2003 Elsevier B.V. All rights reserved.