Lithiated Sn oxide films, denoted LixSnO2, were produced by reactive rf magnetron sputtering of Sn and electrochemical post‐treatment. Optical transmittance and Mo¨ssbauer spectra were recorded for progressively increased lithiation. Increasingxfrom zero to ∼0.1 did not have any significant effect on the optical data or Mo¨ssbauer spectra, and it is likely that the lithium is located in internal double layers in the film. Further increasingxfrom ∼0.1 to ∼0.2 yielded significant transmittance drops and Mo¨ssbauer spectra unambiguously showing a conversion Sn4+→Sn2+. Hence the optical absorption can be reconciled with intervalency transitions as in other cathodically coloring electrochromic oxides. Electrocrystallization appeared to dominate the electrochemistry atx≳0.2. ©1996 American Institute of Physics.