The toxicity and fate of nanoparticles (NPs) have been reported to be highly dependent on the chemistry of the medium, and the effects of phosphate have tended to be ignored despite the wide existence of phosphate contamination in aqueous environments. In the present study the influence of phosphate on the dissolution and microstructural transformation of ZnO NPs was investigated. Phosphate at a low concentration rapidly and substantially reduced the release of Zn(2+) into aqueous solution. Synchrotron X-ray absorption spectroscopy and X-ray diffraction analysis reveal that interaction between ZnO NPs and phosphate induced the transformation of ZnO into zinc phosphate. Transmission electronic microscopy observation shows that the morphology of the particles changed from structurally uniform nanosized spherical to anomalous and porous material containing mixed amorphous and crystalline phases of ZnO and zinc phosphate in the presence of phosphate. To our knowledge, this is the first study in which the detailed process of phosphate-induced speciation and microstructural transformation of ZnO NPs has been analyzed. In view of the wide existence of phosphate contamination in water and its strong metal-complexation capability, phosphate-induced transformations may play an important role in the behaviors, fate, and toxicity of many other metal-based nanomaterials in the environment.