Zn-substituted Ca hydroxyapatites were synthesized by precipitation method under the specific conditions (pH 8, 90 degreesC) and their structural properties were investigated. The substituting limit of Zn was estimated, at about 15 mol%. The lattice parameter a decreased up to 5 mol% Zn, and started to increase over 5 mol% Zn. The lattice parameter c monotonously decreased with increase in Zn fraction. The increase in lattice parameter a for higher Zn fraction was ascribed to increasing amount of lattice H2O which substituted for OH sites in the apatite structure. The lattice H2O in the Zn-substituted apatites was lost by the heat treatment at 400 degreesC. As a result, both the lattice parameters a and c of the heat-treated apatites decreased with increasing Zn fraction. Only the difference in ionic radius between Zn2+, (0.074 nm) and Ca2+ (0.099 nm) was decisive to the change in both lattice parameters after the heat treatment at 400 degreesC. (C) 2004 Elsevier Ltd. All rights reserved.