Electric birefringence and streaming dichroism measurements of catena-mu-2,5-dihydroxy-p-benzoquinonatonickel (II) [dhqNi(II)] aqua sol suggested that the average length of the sol particles was ca. 380 nm and the axial ratio was 2.0-2.3 as the prolate spheroid. The average particle length in the scanning microscopic images was 370 +/- 60 nm and the axial ratio 2.3 +/- 0.6, This aqua colloid was decomposed by UV irradiation under an O-2 gas atmosphere, but under a N-2 gas atmosphere its decomposition was depressed. Furthermore, addition of sacrificial reagents such as L-ascorbic acid depressed the decomposition. These facts suggested that the photogenerated electron originating from the irradiated dhqNi(II) colloidal particle reduced O-2, while the positive hole on the colloid oxidized dhqNi(LT) itself. It was found that the reduction product from O-2 reduced Cr6+ (in Cr2O72-) to Cr3+, and the oxidized dhqNi(II) oxidized I- to I-2/2. Therefore, H2O2 formation was proposed as the photochemical reduction product.