The phase diagram for the ternary system CaO-As2O5-H2O at 23degrees +/-1degreesC is plotted. The calcium arsenates represented are Ca(H(2)ASO(4))(2), CaHASO(4)(.)H(2)O, Ca5H2(AsO4)(4)(.)9H(2)O, Ca-3-(ASO(4))(2)(.)xH(2)O (x = 3(2)/(3) and 4(1)/(4)), Ca-5(ASO(4))(3)OH (apatite), and Ca-4(OH)(2)(AsO4)(2)(.)4H(2)O. The solubility data indicate the following invariant points: Ca(H(2)ASO(4))(2)-CaHASO(4)(.)H(2)O-solution, CaHASO(4)(.)H(2)O-Ca5H2,(AsO4)(4)(.)9H(2)O-solution, Ca5H2-(AsO4)(4)(.)9H(2)O -Ca-3(AsO4)(2)(.)4(1)/4H2O-solution, Ca-3(AsO4)(2)(.)3(2)/3H2O-Ca-4(OH)(2)(AsO4)(2)(.)4H(2)O-solution, and Ca-4(OH)(2)- (AsO4)(2)-4H(2)O-Ca(OH)(2)-solution. The tricalcium arsenate Ca-3(AsO4)(2)(.)3(2)/3H2O is shown to be the only hydrate to dissolve congruently.