||In this study, oscillatory flow photocatalytic reactor (OFPR) was used to degrade persistent organic - salicylic acid (SA) under hydrogen peroxide (H2O2) assisted photocatalytic oxidation with UVC irradiation. Oscillatory flow mixing can efficiently produce uniform dispersion of both anatase and rutile phase titanium dioxide (TiO2) of different sizes. Under same amount of catalysts, the shorter particle spacing of nano-size TiO2 and catalyst aggregation may lead to strong UV scattering; hence UV absorption on TiO2 is reduced. For the rutile phase TiO2 which aggregated particle size is larger than a micrometer, local turbulence was achieved on the TiO2 surface when the critical local Reynolds number, Rec = A0ωDparticle/V exceeded 0.1. Under such conditions, mass transfer in the OFPR was enhanced. Optimum photocatalytic activity for selected TiO2 was achieved when the reactions were conducted with catalyst concentrations at 0.5 g/L for Aldrich rutile and 0.02 g/L for Degussa P25 respectively. The rate of SA degradation was similar for the two catalysts; but the mineralization rate for micro-aggregated size Aldrich rutile TiO2 was higher than nano-aggregated size P25 when both catalysts received the same amount of UV flux. Reaction with micro-aggregated size TiO2 was enhanced by the mass transfer due to local turbulence caused by oscillatory flow mixing. The photocatalytic reaction can be divided into two stages of development. In the first stage, salicylic acid was degraded to intermediates by hydroxyl radicals generated from photolysis of H2O2. The amount of strong UV absorbing intermediates reduced the UV flux reaching the reactor wall. In the second stage, the intermediates were mineralized to carbon dioxide and water. The degradation was enhanced by the photocatalytic reaction on the catalyst. The first stage is the rate-limiting step while the second stage is mass transfer-limiting. lnvestigation of reaction intermediates from the degradation of salicylic acid was carried out using LC/MS. Dihydroxybenzoic acid and trihydroxybenzoic acid were found to be the primary intermediates; both are highly UV-absorbing organic compounds.