Cuprous oxide (Cu2O) nanoparticles of 35 nm in crystal size have been successfully synthesized via electrochemical method in alkali NaCl solutions with copper as electrodes and K2Cr2O7 as additive. Photocatalytic degradation of methyl orange (MeO) in aqueous Cu2O solution was investigated under either ultraviolet (UV) light or sunlight. X-ray diffraction (XRD), transmission electron microscopy (SEM), Fourier transformation infrared spectroscopy (FT-IR), ultraviolet-visible spectroscopy (UV-vis) and X-ray photoelectron spectroscopy (XPS) were introduced to characterize the samples. The results indicate that electric current shows no obvious effect on the growth Of Cu2O nanocrystals and that 97% of MeO can be decolorized under UV irradiation for 2 h or under sunlight for 3 It when amount Of Cu2O is 2 g/L. Recycling use of the catalyst revealed that Cu2O still has a high photocatalytic efficiency when repeatedly used for four times. Cu2O nanoparticles still kept its cubic crystal phase, but fractionally oxidized to be CuO after the photocatalysis. Compared with the original Cu2O nanoparticles, there has 1 eV shift of Cu 2p electron and 1.6 eV shift of Cu Auger signals for the Cu2O Powders after four times photocatalysis. Some new peaks can also be observed at 401.1, 237.4 and 170.2 eV in the Cu2O powders after photocatalysis. (C) 2006 Elsevier Ltd. All rights reserved.