The photocatalytic mineralization of nitrobenzene, nitrosobenzene, phenylhydroxylamine, aniline and 4-nitrosophenol has been investigated in aerated conditions. In slightly acidic TiO2 suspensions, all these compounds proved to undergo quantitative formation of carbon dioxide, whereas nitrogen is converted into nitrate and ammonium ions.The mechanism of ammonium ion release is suggested. Ammonium ions are not generated by reduction of nitrate or nitrite ions, but directly from partly oxidized and unstable intermediates, such as, for example, quinonemonoimine and aliphatic imines. Ammonium ions are formed from nitro-and nitroso-derivatives through reduction of the nitrogen-containing substituent. The reductive pathway prevents detachment of nitro-and nitroso-groups as either nitrate or nitrite ion from the organic moiety. Quite specularly, from phenylhydroxylamine, aniline and aminophenol, ammonium ions are released by means of a partially oxidative pathway. Thus, the temporal evolution of the [NO3-]/[NH4+] concentration ratio, besides other experimental conditions (pH, photon flux, reactant concentrations), depends largely on both the initial oxidation state of the nitrogen and the nature of the starting organic substrate (e.g. 4-nitrosophenol vs. nitrosobenzene).