The photocatalytic oxidation of 2-chloroethyl ethyl sulfide (2-CEES, a simulant for mustard gas) on high area TiO2-SiO2 powder has been studied by transmission infrared spectroscopy. It has been found that the photocatalytic oxidation of 2-CEES occurs on the surface of TiO2-SiO2 at 200 K with an appreciable rate, causing 40-50% depletion of the initial surface coverage before the photoreaction slows down appreciably. In addition to 2-CEES, we have also studied the photooxidation of diethyl sulfide (DES), as a comparison, to test the role of Cl substitution in the 2-CEES molecule. With both 2-CEES and DES, partially and fully oxidized products are observed. By comparison of the spectral developments during the photooxidation of 2-CEES and DES, it appears that the presence of the chlorine substituent in 2-CEES is not strongly influential in the photooxidation sequences for this molecule. The 2-CEES molecule is hydrogen-bonded to the Si-OH groups by both the Cl and the S moieties, and photooxidation leads to loss of the spectral features caused by hydrogen bonding as the 2-CEES molecule is destroyed.