The purpose of this study was to determine the maximum substitution of magnesium, Mg2+, for calcium, Ca2+ sites in a beta-tricalcium phosphate (Mg-beta-TCP) powder using the Rietveld method. Ca-deficient apatite with a theoretical 0.2 Mg/Ca ratio was synthesized and the resultant powder sintered at 1000 degrees C for 4 h. The maximum Mg2+ substitution of nearly 15 mol% in beta-TCP structure was deduced from the occupancy factors. The analysis indicated that the maximum substitution of Mg2+ on the M(4) and M(5) sites was, approximately, 4.21 and 10.72 mol%, corresponding to the Ca-2.61(Mg1(0.28), Mg2(0.11))(PO4)(2) stoichiometric formula. A small displacement of the Ca atoms position was observed around the Mg1(5)PO4 octahedral, and their increasing shift to the PO4 triangles corners. (C) 2009 Elsevier B.V. All rights reserved.