The effect of Ca2+ on phosphate adsorption hydrous zirconium oxide (HZO) was studied. Adsorption data well fitted to Langmuir and pseudo-second-order kinetic models. Phosphate adsorption was due to inner-sphere complexing mechanism. Coexisting Ca2+ enhanced phosphate adsorption onro HZO over a pH range of 6–9. Zr-P-Ca complex formation played an important role in enhanced adsorption by Ca2+.