The electrochromic reaction of titanium oxide films prepared by r.f. sputtering was studied. Chronoamperometric experiments associated with transmittance spectra in LiClO4-propylene carbonate solutions were carried out and compared with the optical properties of titanium oxide films with different stoichiometries. It was verified that the colour changes produced by stoichiometry deviation and by electro-intercalation are similar, because the maximum absorption bands in both cases are centred at the same wavelength and the molar absorption coefficients are similar; these observations can be explained by the mixed valence theory deduced for non-stoichiometric metallic oxides. The analysis of the j vs. E and [d(OD)/dt] vs. E potentiodynamic profiles is an interesting method for studying the kinetic mechanisms of electrochromic reactions without interference from parallel reactions.