Illumination of aqueous suspensions of Pt/TiO2 semiconductor photocatalyst in the presence of [Ru-III(edta)(H2O)](-) resulted in the reduction of hydrazine to ammonia. A working mechanism involving the formation of a [Ru-III(edta)(N2H5)] species (adsorbed onto TiO2 surface) which undergoes two-electron transfer reduction followed by cleavage of the N-N bond of coordinated hydrazine has been suggested.