The photoelectrochemical behavior of passive films on Co has been studied as a function of the polarizing voltage and electrolyte composition. Passive layers formed at 0 V (standard hydrogen electrode, SHE! in 0.1 M NaOH consisted of Co(OH)(2), whose bandgap value has been found to be 1.85 eV. Higher bandgap values (2.75 eV) have been measured for passive films formed in borate buffer at 0 V (SHE), which are mainly consist of CoO. The E-g values have been related to the film composition on the basis of a correlation between the bandgap of passive films and the electronegativity of their constituents. (C) 2005 The Electrochemical Society.