A mixed-valence tin oxide, (Sn(2+))2(Sn(4+))O4, was synthesized via a hydrothermal route. The Sn3O4 material consisted of highly crystalline {110} flexes. The Sn3O4 material, when pure platinum (Pt) was used as a co-catalyst, significantly catalyzed water-splitting in aqueous solution under illumination of visible light (λ > 400 nm), whereas neither Sn(2+)O nor Sn(4+)O2 was active toward the reaction. Theoretical calculations have demonstrated that the co-existence of Sn(2+) and Sn(4+) in Sn3O4 leads to a desirable band structure for photocatalytic hydrogen evolution from water solution. Sn3O4 has great potential as an abundant, cheap, and environmentally benign solar-energy conversion catalyst.