Tantalum(V) oxide (Ta₂O₅) powders were synthesized by a solvothermal reaction of tantalum pentabutoxide (TPB) in toluene at 473–573 K in the presence of water. Ta₂O₅ powder with a large surface area (>200 m² g⁻¹) was obtained at 473 and 523 K, and its X-ray diffraction patterns showed no clear peaks of crystallites, while crystalline β-phase Ta₂O₅ was obtained in the solvothermal process at 573 K. Post-calcination of the amorphous Ta₂O₅ at 973 K also induced crystallization into the β-phase. These amorphous and crystalline Ta₂O₅ particles were platinized and then used for photocatalytic production of hydrogen from 2-propanol in aqueous solution under deaerated conditions by irradiation at a wavelength >300 nm. As-synthesized amorphous and crystalline β-phase samples exhibited negligible rates of hydrogen evolution, whereas crystalline β-phase samples formed by post-calcination showed higher activity. Calcination of amorphous Ta₂O₅ prepared by hydrolysis of TPB under atmospheric pressure also increased the rate, which is, however, ca. one third of that of Ta₂O₅ obtained ia the solvothermal route. Mineralization of acetic acid under aerated conditions was conducted as a test reaction for photocatalytic decomposition of organic compounds. Also, in this reaction system, bare (not platinized) solvothermal Ta₂O₅ exhibited a higher rate of CO₂ formation than other Ta₂O₅ samples. Photoirradiation at >200 nm onto an aqueous 2-propanol suspension of platinized Ta₂O₅ under deaerated conditions yielded hydrogen and acetone in an early stage, but CO₂ was liberated by prolonged irradiation, suggesting that decomposition of acetone to CO₂ proceeds even under deaerated conditions.