The mechanical effects of the intercalation processes in electrochromic WO3 thin films are reported here and discussed. In particular the electrochemical insertion of H, Li and Na ions in WO3 was studied by means of laser beam deflection method (LBDM). Linear changes of WO3 stress were observed for small amounts of the inserted charge and linearity was always associated with a reversible mechanical behavior of thin films. An explanation in terms of homogeneity of the insertion is given. An analogous trend was also verified in the absorbance vs. charge curves. As a consequence the constancy of the electrochromic efficiency values for the three different ions was found. In this way it was possible to determine a full reversible behavior for MxWO3 in the composition range O less than or equal to x less than or equal to 0.2. The onset of new phases formation when x exceeded previous upper limit, was observed during Li and Na intercalation. Such transitions brought about the loss of optical and mechanical reversibility. During prolonged hydrogen insertion a reversible slope inversion occurred in stress curve so that it was necessary to take into account different possible mechanisms of the WO3 electrochromic reaction.