The photocatalytic reduction of nitrite and nitrate ions to ammonia has been studied over ZnO, ZrO2, Fe2O3 and Fe3O4 oxide semiconductors. Many parameters such as the position of the conduction band edge of the semiconductor, the metal-semiconductor junction, light absorption capacity of the semiconductor, adsorption of the reagent species and pH of the medium used influence the yield of ammonia and hence one can not a priori predict the relative activity of these systems on the basis of band edges alone. The resulting photoactivity is a balance of the above said factors some of which play contrasting roles.