It has been reported that cobalt phosphate (CoP) deposited on TiO₂ is beneficial to the photoelectrochemical (PEC) oxidation of water in a phosphate solution, but detrimental to the photocatalytic (PC) oxidation of phenol even in a phosphate solution. Herein, we report amorphous nickel phosphate (NiP) deposited on TiO₂ being more active than TiO₂ in a phosphate-free solution. This was observed not only from the PEC oxidation of water, but also from the PC oxidation of phenol under air, and from the PC reduction of water to H₂ under N₂ in the presence of methanol, respectively. During the PC reactions under air and N₂, however, the suspension of NiP/TiO₂ changed its colour from white to pale yellow and dark grey, respectively. Such colour changes were also observed from CoP/TiO₂ under similar conditions. Through a spectral and thermodynamic analysis, it is proposed that under air, NiP is oxidized to a high valence Ni species, and then regenerated through water and phenol oxidation, while NiP under N₂ is reduced to metallic Ni, followed by proton reduction.