The effect of different Mg concentrations on the sorption of phosphorous by CaCO3 was studied at different P concentrations, two temperatures and three incubation periods. Results indicated that, at all concentrations, Mg ions reduced P sorption by CaCO3. The effect of Mg2+ ions was more conspicuous at high than at low P levels and with increasing time of incubation and temperature. The effect of Mg at reducing P sorption may be ascribed to the formation of ion pairs with either HCO3– or CO3= and inhibiting the nucleation and crystal growth of P precipitates.