Decomposition of environmentally persistent perfluorooctanoic acid (PFOA) in aqueous suspensions of a TiO2 photocatalyst, with use of medium-pressure ultraviolet (MPUV) lamp irradiation alone, was examined under atmospheric pressure. Compared to direct photolysis, TiO2 photocatalysis led to efficient PFOA decomposition and production of CO2 and F-. PFOA decomposition followed pseudo-first-order kinetics, with observed rate constants of 1.3 x 10(-2) and 8.6 x 10(-2) dm(3) h(-1) in direct photolysis and TiO2 photocatalysis, respectively. The latter (photocatalytic) rate constant is 5-100 times greater than those obtained in other published research on the photocatalytic decomposition of PFOA. In the proposed decomposition pathway, PFOA molecules adsorb onto the TiO2 surface according to adsorption equilibrium in an aqueous suspension and could be easily decomposed by holes and radicals generated by MPUV lamp irradiation. Under the present reaction conditions, a narrow region of TiO2 concentrations around 1.5 wt % showed the maximum extent of PFOA decomposition, CO2 formation, and F- formation. The optimum rate can be attributed to a trade-off between an increase of photon absorption by TiO2 and a decrease in UV penetration below the surface of the TiO2 suspension. In the 1.5 wt % TiO2 photocatalyst concentration, 5 mM PFOA was almost totally decomposed during 4 h of MPUV lamp irradiation under atmospheric pressure.