The diffusion coefficients for lithium ions in tungsten oxide and titania-doped tungsten oxide have been measured using the AC impedance spectroscopy technique. Films were prepared using both sol-gel dip coating and DC magnetron sputtering at various substrate temperatures. The diffusion coefficients for sol-gel deposited films post-processed at higher temperatures (400 degrees C) are lower than for lower processing temperatures (300 degrees C). For sputtered films the diffusion coefficients for higher substrate temperatures are similar to those for lower substrate temperatures. While there is a weak dependence of diffusion coefficient on level of charge injection, there is a strong correlation between the optical modulation of the films and the diffusion coefficient, with significantly larger optical modulation for films with larger diffusion coefficients. A possible explanation for this correlation in terms of film microstructure is discussed.