Vanadium-doped titania is found to be a better photocatalyst for gas phase photo-oxidation of ethylene than nano titania. In situ FTIR studies were undertaken to elucidate the mechanistic pathway for ethylene oxidation on these two catalyst surfaces. Vanadium doping leads to formation of more chemisorbed hydroxyl species, which makes it a better photocatalyst. The labile hydroxyls which were responsible for the reduction of V(5+) to V(4+) during the process of calcination were also ascertained. The ethylene decomposition occurs via formation of ethoxy groups, transformed to acetaldehyde or enolates, subsequently to acetates/formates, and then to CO(2). The enolates were more stabilized on the TiO(2) surface, leading to formation of formates along with the acetates. On vanadium-doped TiO(2), acetaldehyde was more stabilized than its enol tautomer, leading to the formation of labile acetic acid and acetates. The formation of the labile acetic acid, adsorbed acetates, and the adsorbed acetate -M salts led to easier oxidation of them to provide higher yield of CO(2). The higher positive charge density over Ti in Ti(0.95)V(0.05)O(2) with respect to nano TiO(2) makes the acetate (stronger nucleophile) a more stable intermediate on it.