Gas phase photocatalytic oxidation (PCO) of unsymmetrical dimethyl hydrazine (UDMH) in a batch reactor using TiO2 as the photocatalyst was studied with in situ FTIR method. Carbon dioxide, water, nitric acid and nitrogen were detected as the ultimate PCO products of UDMH. Adsorbed N2O species were detected as the main surface intermediates. The formation of the extremely toxic intermediate, nitrosodimethylamine (CH3)(2)NNO, was not observed. The reaction mechanism based on detected products distribution, kinetics analysis and previously published data is discussed. Long-term experiments were carried out to determine the photocatalyst stability in PCO of UDMH: the catalyst exhibited stable performance; the rate of deactivation was low due to transformation of nitrogen mainly to N-2. Only 10% of UDMH nitrogen was transformed to HNO3 in adsorbed form. (c) 2007 Published by Elsevier B.V.