Density functional calculations at the B3LYP/6-311G**//B3LYP/6-311G level of theory were used to study the initial process of ethylene degradation on the TiO2 photocatalyst by adopting the dimeric titanium structure Ti2O6H4 as a model of the catalyst surface. Adsorption energies of water and ethylene were calculated to be 31.9 and 20.4 kcal mol(-1). The photogenerated OH radical does not desorb from the catalyst surface and the further reaction with ethylene proceeds since the adsorption energy was estimated to be 39.3 kcal mol(-1). Our calculation also indicated that under steady illumination, ethylene directly attacks the OH radical bound to the TiO2 surface even though the surface has vacant sites available for ethylene adsorption.