The photooxidation of phenol in the presence of either cyanide or sulphide ions at different initial concentrations in aqueous suspensions of ZnO was studied. It was observed that the presence of sulphide lowers the yield of phenol oxidation. Sulphide is also photooxidized maximally, at low phenol concentrations, the amount of sulphide oxidized being greater than in the absence of or in high concentrations of phenol. On the other hand, in phenol-cyanide solutions a strong competition occurs that decreases the oxidation yield of both phenol and cyanide. From the results of this work, it can be assumed that sulphide is oxidized by heterogeneous reaction with the photogenerated holes in the ZnO, while cyanide and phenol are oxidized by means of both photogenerated holes and OH radicals. However, at low concentrations, the oxidation of phenol takes place preferentially by homogeneous reaction with OH radicals. © 1989.