A method and apparatus for mineralizing organic contaminants in water or air provides photochemical oxidation in a unique two-phase or three-phase boundary system formed in each pore of a TiO.sub.2 membrane in a photocatalytic reactor. In the three-phase system, gaseous oxidant, liquid contaminant, and solid semiconductor photocatalyst having a metal catalyst disposed thereon meet and engage in an efficient oxidation reaction. The porous membrane has pores which have a region wherein the meniscus of the liquid varies from the molecular diameter of water to that of a capillary tube resulting in a diffusion layer that is several orders of magnitude smaller than the closest known reactors. The photocatalytic reactor operates effectively at ambient temperature and low pressures.