High-resolution FT-IR spectroscopy combined with quantum chemical calculations was used to study the chemistry of OCS-disproportionation over the reduced surface of isotopically labelled, nanocrystalline TiO2. Analysis of the isotopic composition of the product gases has revealed that the reaction involves solely OCS molecules fromthe gas-phase. Using quantumchemical calculations we propose a plausible mechanistic scenario, in which two reduced Ti3+ centres mediate the reaction of the adsorbed OCS molecules.