AbstractComposites comprised of calcium deficient hydroxyapatite (CA/P = 1.5) and gelatin form in about 4 h at 38°C. The solid reactants used are CaHPO4and Ca4(PO4)2O and the liquid reactant is gelatin solution. The reaction forming hydroxyapatite occurs in several steps. Based on the measurement of heat liberation, there is an initial period where the reaction rate is low. This is followed by a period of constant reaction until the reaction is complete. Analysis of the variation in pH shows that hydroxyapatite formation occurs at a steady state pH near 8.2. Analyses of the evolution of crystalline phases indicates that Ca4(PO4)2O is consumed prior to complete reaction resulting in a phase assemblage of hydroxyapatite (CA/P>1.5) and CaHPO4. Further reaction results in compositional adjustments in hydroxyapatite as CaHPO4is consumed. Microstructurally, the composites which form consist of clusters of hydroxyapatite embedded in a gelatin network. The presence of gelatin does not influence the morphology of the hydroxyapatite which forms nor does it retard its rate of formation. © 1994 John Wiley&Sons, In