The photodegradation of methyl orange was found to take place very efficiently using hollow Au nanocages which are known to have remaining Ag on their interior walls which can be oxidized to Ag2O. The degradation rate is found to be more efficient than photodegradation reaction using semiconductor nanomaterials, such as TiO2 and ZnO. The reaction rate is found to increase by increasing the degree of Ag oxidation on the interior wall of the nanocages prior to the reaction and is a function of the nanocavity size and the pore density of the nanocage walls. As the cage size varies, it is found that the photocatalytic rate increases and then decreases with a maximum rate at nanoparticle size of 75 nm with a medium pore density-in the walls. All these results suggest that the catalysis is occurring inside the cavity, whose interior walls are covered with the Ag2O catalysts. Similar to the mechanism proposed in the degradation by the other semiconductors, we propose that the photodegradation mechanism involves the formation of the hydroxyl radical resulting from the photoexcitation of the Ag2O semiconductor. The observed results on the rate are discussion in terms of (1) the surface area of the inner wall covered with Ag (Ag2O), (2) the density and size of the pores in the walls, and (3) the cavity size of the nanoparticles.