CuO powders with a high specific surface area are shown to be able to produce H2O2 in aqueous solution under simulated light irradiation. The highest rate of peroxide production was observed under mild experimental conditions using O2 and a large surface area photocatalyst CuO irradiated with a solar simulator having light intensities between 60 and 90 mW/cm2. The CuO employed had a specific surface area (SSA) of 64.8-70.1 m2/g and was prepared in a tubular furnace by controlled thermal decomposition of precipitated copper oxalate. The CuOparticles produced were 1 μmcubes with primary particles around 15 nm. No peroxide was produced under the same conditions with commercial CuO, with SSA 200 times lower. The CuO synthesized during this work was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), specific surface area [Brunauer-Emmett-Teller (BET)], porosity, and X-ray photoelectron spectroscopy (XPS). From XPS, it was observed that only CuII was present in the unused and used CuO. This indicates that the redox transient species involving other Cu oxidation states disappear very fast during the reaction, regenerating CuII during H2O2 production. Diverse experiments provided some evidence for the possible interfacial reaction mechanism leading to H2O2, following the initial step of O2-‚ formation on the CuO surface under irradiation with photons, with energies exceeding the band gap of CuO. A photocatalyzed degradation of a concentrated 4-chlorophenol (4-CP) solution was observed under solar-simulated light in the presence of CuO.