Cobalt oxide films have been prepared by electrodeposition. Such films have been tailored in two ways. Firstly, films may contain other metal oxides, thereby increasing the ionic conductivity within the solid. Cr, Fe, Mn, Ni, W or Zn oxides were used to this end. Secondly, the composition of the film may be "fine tuned" since mole fractions of constituent oxides depend on the voltage used for deposition, V-a. Cobalt oxide underwent an electrochromic transition between an intense brown colour at anodic potentials and was green when the underlying electrode was cathodic. Samples containing other oxides showed enhanced ionic conductivity, eg the chemical diffusion coefficient D increased from 2.3 x 10(-8) cm(2)s(-1) for CoO in 0.1 M NaOH to 48.7 x 10(-8) cm(2)s(-1) for Co/W oxide. The electrochromic colour formed was more intense for doped cobalt oxide films than for CoO, Co/Ni oxide changing from pale green to black for example. It is suggested that mixing of oxide constituents is intimate.