As part of a study involving sorption experiments with actinide ions sorbed onto phosphate compounds and to understand the sorption mechanisms, we have first synthesized the following three solids: ZrP2O7 (zirconium diphosphate), Zr2O(PO4)(2) (zirconium oxophosphate) and Th-4(PO4)(4)P2O7 (thorium phosphate diphosphate). These compounds have been characterized using X-ray powder diffraction, IR spectroscopy and EPMA. The specific surface areas, measured by the N-2-BET method, are 5.5 +/- 0.1, 0.9 +/- 0.2 and 1.2 +/- 0.2 m(2) g(-1) respectively. The average grain size of the powdered samples is around 10 mu m. The pH(IEP) (pH corresponding to the isoelectric point) determined by electrokinetic measurements is 3.6 for ZrP2O7, 4.0 for Zr2O(PO4)(2) and 6.8 for Th-4(PO4)(4)P2O7. By simulation of the titration curves obtained for the three solids, we have determined, using the FITEQL code (constant capacitance model), the surface acidity constants using a sorption site number close to 7 sites nm(-2) for the zirconium diphosphate, 7.5 sites nm-2 for the zirconium oxophosphate and 4 sites nm(-2) for the thorium phosphate diphosphate.