The gas-phase heterogeneous photocatalytic oxidation of diethyl sulfide (DES), a simulant for chemical agent mustard gas, was investigated in a batch reactor using detection of gaseous products and FT-IR idenfification of surface species. Acetaldehyde and ethylene were detected as gaseous intermediates and diethylsulfone and carboxylates were detected as surface intermediate products. It was found that DES is oxidized completely and the final products of its oxidation are carbon dioxide, water and surface sulfate species. Deactivation of the TiO2 photocatalyst was evidenced by increased time of DES complete mineralization in consecutive treatment of fixed amount of DES in the batch reactor. Carbonate and sulfate species were detected on the TiO2 surface after complete DES oxidation by means of Fr-IR diffuse reflectance spectroscopy. These structures turned out to be responsible for TiO2 deactivation.