The formation of biocompatible organic-inorganic composites by reactions between tetracalcium phosphate (Ca4(PO4)2O, TetCP) and the biomedical polymer poly(vinyl phosphonic acid) (PVPA) is described. Composites were prepared by hot pressing mixtures of these powders at 80 kpsi and 300 degrees C for 30 min. Composite formation was investigated depending on the proportions of reactants and the processing route used. Two inorganic phases were produced as a result of the acid-base reaction between TetCP and PVPA: hydroxyapatite (Ca10(PO4)6(OH)2, HAp) and anhydrous dicalcium phosphate (CaHPO4, DCPA). The later phase preferentially formed at lower TetCP/PVPA ratios while the amount of HAp increased with increasing TetCP/PVPA ratio. The reactions appear to start with the softening of the polymer when heated to T > Tg. The flowing polymer surrounds the TetCP grains permitting the TetCP to initially form DCPA crystallites in a matrix of the Ca salt of the polymer. When H2O is added prior to pressing, the DCPA produced reacts with the remaining TetCP forming HAp.