CdO2 nanocrystals with different sizes were synthesized via the hydrothermal reaction of 3CdSO(4)center dot 8H(2)O and H2O2 in 1.25-6.25 vol.% ammonia solutions at 100-140 degrees C for 12h. The resultant products were characterized by powder X-ray diffraction, transmission electron microscope, thermal gravimetric and differential scanning calorimetry, and UV-vis absorption spectra. It was concluded that the resultant products were pure cubic phase CdO2 nanocrystals, and they would decompose at temperatures higher than 180 degrees C. In addition, degradation of methyl orange in aqueous solution (20.0 mg/l) was carried out with the CdO2 nanocrystals as photocatalyst under ultraviolet light irradiation. The experimental results showed that even a little amount (0.2 g) of as-prepared CdO2 nanocrystals could catalyze degradation of 500 ml methyl orange solution above 99% after 5 h of illumination, and smaller size made for higher photocatalytic activity of CdO2 nanocrystals. (C) 2008 Elsevier B.V. All rights reserved.