Photocatalytic activity under visible light irradiation was generated by doping a small amount of Co2+ ions into TiO2 particles. Nanometer-sized particles with the composition xCoO-(100 - x) TiO2 (xCo/TiO2; 0 less than or equal to x less than or equal to 6.0) were prepared from TiOSO4.4.5H(2)O and Co(CH3COO)(2).4H(2)O in an ethanol-water solution containing NaOH under reflux, The photoreactivity of xCo/TiO2 was evaluated by the rate of CH3CHO degradation. The Co2+ ion doping (0.01 less than or equal to x less than or equal to 0.27) not only enhanced the photocatalytic activity under ultraviolet-visible (lambda > 300 nm) light irradiation but also induced the visible light (lambda > 400 nm) response. The highest photocatalytic activities were obtained at x = 0.03 for both irradiations.