The oxygen isotopic composition of dissolved inorganic phosphate (δ¹⁸O_p) in many aquatic ecosystems is not in isotopic equilibrium with ambient water and, therefore, may reflect the source δ¹⁸O_p. Identification of phosphate sources to water bodies is critical for designing best management practices for phosphate load reduction to control eutrophication. In order for δ¹⁸O _p to be a useful tool for source tracking, the δ¹⁸O_p of phosphate sources must be distinguishable from one another; however, the δ¹⁸O_p of potential sources has not been well characterized. We measured the δ¹⁸O _p of a variety of known phosphate sources, including fertilizers, semiprocessed phosphorite ore, particulate aerosols, detergents, leachates of vegetation, soil, animal feces, and wastewater treatment plant effluent. We found a considerable range of δ¹⁸O_p values (from +8.4 to +24.9‰) for the various sources, and statistically significant differences were found between several of the source types. δ¹⁸O_p measured in three different fresh water systems was generally not in equilibrium with ambient water. Although there is overlap in δ¹⁸O_p values among the groups of samples, our results indicate that some sources are isotopically distinct and δ¹⁸O_p can be used for identifying phosphate sources to aquatic systems.