Interactions of pyrophosphate (PP) with hydroxyapatite (HAP), brushite, and calcium pyrophosphate (CaPP) have been studied using electrokinetic and ESCA measurements. PP adsorbs on HAP and brushite even when their surfaces are highly negatively charged. Importantly, the HAP zeta potential curves as a function of PP concentration show a precipitous drop in the low concentration range. Also, curves at different pH values show a progressive shift in the potential toward that of CaPP. These observations suggest the formation of CaPP on HAP surface and this has been further confirmed by ESCA studies. Thus, adsorption on HAP appears to involve a surface precipitation or chemisorption followed by surface precipitation or surface reaction phenomenon. It is also suggested that in the case of brushite the dissolved calcium from the mineral competes for the PP and in turn reduces its adsorption.