The transformation beta ->alpha in Mg-substituted Ca-3(PO4)(2) was studied. The results obtained showed that, contrary to common belief, there is, in the system Mg-3(PO4)(2)-Ca-3(PO4)(2), a binary phase field where beta+alpha-Ca-3(PO4)(2) solid solutions coexist. This binary field lies between the single-phase fields of beta- and alpha-Ca-3(PO4)(2) solid solution in the Ca-3(PO4)(2)-rich zone of the mentioned system. In the light of the results and the Palatnik-Landau's Contact Rule of Phase Regions, a corrected phase equilibrium diagram has been proposed. The practical implications of these findings with regard to the synthesis of pure alpha- and beta- Mg-substituted Ca-3(PO4)(2) powders and to the sintering of related bioceramics with improved mechanical properties are pointed out.