The changes that the electrochemical insertion of ions (M equals H, Li or Na) cause in the optical and mechanical properties of electrochromic tungsten bronzes, MxWO3, are reported. The commonly accepted assumption that the electrons and cations have separate effects, namely on the optical properties and film volume (stress), respectively, does not properly describe the results that are obtained with the three different cations. Differences between the cations are explained in terms of cation diffusion in a solid medium. Particular attention is given to the use of the laser beam deflection method, in conjunction with in- situ optical measurements, to characterize electrochromic thin films.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.