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 V5+ to V4+ 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 CO2. The enolates were more stabilized on the TiO2 surface, leading to formation of formates along with the acetates. On vanadium-doped TiO2, 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 CO2. The higher positive charge density over Ti in Ti0.95V0.05O2 with respect to nano TiO2 makes the acetate (stronger nucleophile) a more stable intermediate on it.