The behaviour of non-stoichiometric hydroxyapatite (HA) during the calcination in a solid bed was investigated. The structural properties are described in terms of the specific surface area. Calcination led to a significant decrease of the specific surface area by particle coalescence and densification. Hydroxyapatite begins to shrink near 780 degreesC and reaches 97% theoretical density at 1100 degreesC. The specific surface area and density variations are caused both by sintering and chemical reaction. Sintering data from these solids were correlated as a function of time and temperature. The rate of sintering is assumed to obey an Arrhenius equation. These results are compared with a number of literature models describing the mechanism of sintering kinetics using the specific surface area, and a good agreement is observed. The kinetic equation used is based on sintering driven by the curvature gradient in the interparticle neck region associated with initial stage sintering. Then, the decline in specific surface area is accurately described by the empirical equation of the form dS/dt = -B(T)k(b). The changing value of b, also known as the "order" of the reaction, suggests that the diffusion mechanism for loss of surface area may be a function of the temperature. (C) 2003 Elsevier B.V. All rights reserved.