The structure and composition of passive film formed on Zircaloy-4 in pH 8.5 buffer solution was examined qualitatively through the photo-electrochemical analysis of the film. The passive film was found to be an n-type semi-conductor composed of single or duplex layers depending on the film formation potential, polarization time and applied potential. The passive film formed at potentials active to 0 V versus saturated calomel electrode (SCE) was a single layer of hydrous ZrO2 with a band-gap energy of 2.98 +/- 0.29 eV. In contrast, the passive film formed at potentials noble to 0.25 V-SCE was composed of duplex layers with an outer hydrous ZrO2 layer and an inner anhydrous ZrO2 layer with band-gap energy of 4.30 +/- 0.15 eV that is close to that (4.77 eV) of crystalline ZrO2 film formed in air at 400 degreesC. The inner anhydrous ZrO2 layer grew as film formation potential or polarization time increased. By examining the effects of applied potential on the photo-current spectrum for each layer of the passive film formed at 1 V-SCE, the flat-band potential for the outer hydrous oxide layer was determined to be -0.6 V-SCE while that for the inner anhydrous oxide layer was -1.2 V-SCE.