The stabilities of calcium arsenate compounds were established by analysis of suspensions made with varying molar Ca/As ratios. Solution chemistry analyses determined the concentrations of calcium and arsenic and pH. The phases that were shown to form in order of descending pH were Ca 4(OH) 2(AsO 4) 2·4H 2O, Ca 5(AsO 4) 3OH (arsenate–apatite), Ca 3(AsO 4) 2·3 2 3 H 2O, Ca 3(AsO 4) 2·4 1 4 H 2O, Ca 5H 2(AsO 4) 4·9H 2O — ferrarisite, Ca 5H 2(AsO 4) 4·9H 2O — guerinite and CaHAsO 4·H 2O. The analytical concentrations of calcium and arsenic and pH were used in estimating solubility products. The estimated values were then refined through the comparison of the analytical data with calculated K sp values using the computer program PhreeqC. From the refined solubility products, the free energies of formation of the calcium arsenate hydrates were calculated as follows: Ca 4(OH) 2(AsO 4) 2·4H 2O (−4941 kJ/mol), Ca 5(AsO 4) 3OH (−5087 kJ/mol), Ca 3(AsO 4) 2·3 2 3 H 2O (−3945 kJ/mol), Ca 3(AsO 4) 2·4 1 4 H 2O (−4085 kJ/mol), Ca 5H 2(AsO 4) 4·9H 2O — ferrarisite (−7808 kJ/mol), Ca 5H 2(AsO 4) 4·9H 2O — guerinite (−7803 kJ/mol), and CaHAsO 4·H 2O (−1533 kJ/mol). Unlike other solubility studies on arsenate immobilization, this study was the first to consider the complete array of calcium arsenate hydrates that can form and to use the associated ions, CaAsO 4 −, CaHAsO 4 0 and CaH 2AsO 4 + in determining their solubility products.