Abstract: The formation mechanism of free radicals in the photocatalysis decomposition processes for aromatic compounds, p-Chlorophenol,Bromobenzene,Acetophenone and α-Phenylethanol,adsorbed on the surface of Titanium dioxide colloid have been studied by means or spin trapping-ESR method. It is very intcrcsting that the photoreaction mechanism on the colloid surface is not identical with the mechanism in the case without colloid present.The origin of leading to differences is explored.