The photochromic effect of thin amorphous WO3films, i.e., the coloration upon exposing the sample to ultraviolet light, has been investigated by means of optically excited surface plasmons. Due to the high sensitivity of this technique, it was possible to investigate the dynamics of this effect on a scale as short as seconds. Our results demonstrate the dominant role of oxygen during the coloration and bleaching processes. We found clear evidence of an oxygen exchange between the sample and the ambient atmosphere during the photochromic process, which is at variance with the widely accepted double charge injection model. The coloration rate shows the same wavelength dependence as that of the creation rate of electron‐hole pairs formed by optical excitation. The results can be explained by the light‐induced decomposition of the incorporated water.