Hydroxyapatite-biodegradable polymer composites have been formed by a low temperature chemical route. Precomposite structures were prepared by combining alpha-Ca-3(PO4)(2) (alpha-tricalcium phosphate or alpha-TCP) with poly(L-lactic) acid and poly(DL-lactide-co-glycolide) copolymers. The final composite structure was achieved by in situ hydrolysis of alpha-TCP to Ca-9(HPO4)(PO4)(5)OH (calcium deficient hydroxyapatite or CDHAp) either in solvent cast or pressed precomposites. Hydrolysis was performed at 56 degrees C-a temperature slightly above the glass transition of the polymers. The effects of polymer chemistry, composite formation technique, and porosity on hydrolysis kinetics and degree of transformation were examined with isothermal calorimetry, X-ray diffraction (XRD), Fourier transform infrared spectroscopy, and scanning electron microscopy. Calorimetric data and XRD analyses revealed that hydrolysis reactions were inhibited in the presence of the polymers. Isothermal calorimetry indicated the extent of the alpha-TCP to CDHAp transformation in 24 h to be 85% in the solvent cast composites containing PLGA (85:15) copolymer; however, XRD analyses suggested almost complete reaction. The CDHAp formation extent was 26% for the pressed composites containing the same polymer. In the presence of NaCl as a pore generator, 81% transformation was observed for the pressed composites. This transformation occurred without any chemical reaction between the polymer-inorganic components, as determined by Fourier transform infrared spectroscopy. Minimal transformation to CDHAp occurred in composites containing poly(L-lactic) acid. (C) 2000 John Wiley & Sons,Inc.