A common pathway for P-recovery from wastewater is phosphate precipitation as calcium phosphates. In this paper, a thermodynamic model for phosphate precipitation is proposed based on various models of activity coefficients taking into account various calcium phosphate phases which can crystallize in the range of pH to be considered, i.e. both dicalcium phosphate dihydrate (DCPD) and amorphous calcium phosphate (ACP) for pH lower than 7.3 and only ACP for pH higher than 7.3. The parameters include the solubility products of ACP and DCPD species. The observed discrepancy in liquid phase equilibrium constants reported in the literature leads to determination of an uncertainty zone describing the precipitation domain. The results obtained offer interesting possibilities for a further optimization of process operating conditions, i.e. determining Ca/P molar ratio, in order to reduce effluent pH (thus avoiding post-treatment) and, consequently, to maximize reactor efficiency. (C) 2003 Elsevier Science B.V. All rights reserved.