The photocatalytic oxidation of cyanate ions in aqueous solution and in the presence of TiO2 takes place by two different reaction pathways, depending on the solution pH. For pHs lower than 10, the main oxidation product is nitrate instead of N-2, as reported previously. The hydrolysis of cyanate seems to be the reason for this change of mechanism with pH NH4+ is formed by CNO- hydrolysis, and its photo-oxidation gives nitrite and nitrate. The hydrolysis rate controls the speed of the whole process, but the photo-oxidation of CNO- to N-2 appeared to be faster in a strongly basic medium. For values of pH lower than 5, NH4+ is hardly oxidized, remaining stable in solution.