Semiconducting TiO2 is the most studied photocatalyst for organic oxidation by O-2. To accelerate the reaction, a cocatalyst for O-2 reduction or for organic oxidation is often used, but the bifunctional one is rare. Herein we report a spinel CoFe2O4 (CF) efficiently catalyzing O-2 reduction and phenol oxidation on TiO2 in aqueous suspensions at pH 3-11. The composite materials (CF/TiO2) were made by depositing 0-5 wt % CF onto TiO2 through a hydrothermal method. Solid characterization showed that CF nanoparticles (5 nm) homogeneously distributed in CF/TiO2, whereas the TiO2 phase remained unchanged in crystal structure and crystallite size. For phenol oxidation under UV light, CF was nearly not active, but 0.01 wt % CF/TiO2 was more active than TiO2, by approximately a factor of 3.6. Such trend in activity among the catalysts was also observed from the photocatalytic reduction of O-2 to H2O2 from the electrochemical reduction of O-2, and from the photoelectrochemical oxidation of phenol and H2O. An open-circuit potential and photoluminescence measurement suggest that there is an interfacial electron transfer from TiO2 to CF, followed by O-2 reduction. Accordingly, a possible mechanism is proposed, involving CF catalysis for O-2 reduction and phenol oxidation. Then the mutual promotion between electron and hole transfer results in great enhancement in the efficiency of charge separation and hence in the rate of chemical reaction. Because spinel compounds have rich composition and unique structures, they are worthy of being further investigated as cocatalysts of a semiconductor photocatalysis.