Surface properties of photocatalysts are crucial to improve their efficiency in the removal of wastewater pollutants. In this study, phosphate-modified BiPO₄ photocatalysts with different PO₄³⁻/Bi molar ratios were controllably synthesized through a one-pot hydrothermal method. The concentrations of surface hydroxyl groups on BiPO₄ samples can be tuned by adjusting the PO₄³⁻/Bi³⁺ molar ratios. XRD results showed that surface modification by phosphate could boost the crystallinity of BiPO₄, which was beneficial to photocatalytic performance. XPS and IR spectra revealed that PO₄³⁻ modified only the surface of BiPO₄. The optical absorption of phosphate-modified BiPO₄ samples showed an obvious blue shift compared with that of bare BiPO₄. Photocatalytic activities of the as-prepared samples were evaluated by the decomposition of methylene orange (MO) under ultraviolet light and simulated solar-light irradiation. Experimental results demonstrated that the photocatalytic performance of the modified samples significantly improved after the BiPO₄ surface was modified by different amounts of PO₄³⁻. Both an indirect scavenger method and an electron spin resonance method were adopted to determine the production and intensity of free radicals during photocatalytic degradation. Results suggested that MO was removed predominantly through oxidation by OH radicals, as confirmed by theoretical calculations. Moreover, the mechanism of photocatalytic degradation of MO over the as-prepared phosphate-modified BiPO₄ was proposed. Copyright © 2015 The Royal Society of Chemistry.