High-temperature solid state syntheses of monazite powders containing plutonium (III), plutonium (IV) and americium (III) were performed. Resulting powders were characterized by X-ray diffraction. Pu3+PO4 was readily obtained as a single phase by heating a pU(4+)O(2)-NH4H2PO4 mixture under argon atmosphere. Traces of tetravalent plutonium phosphate pu(4+)P(2)O(7) were detected when synthesized under air atmosphere. The incorporation Of (pu(4+), Ca2+) in the monazite structure was investigated under air and argon atmosphere. We showed that pu(4+) is fully reduced in Pu3+ under argon atmosphere whereas, under air, the compound with the formula Pu0.43+Pu0.34+Ca0.32+PO4 was obtained. Pure Am3+PO4 was also synthesized under argon atmosphere. X-ray patterns revealed a complete amorphisation of the monazite structure at a relatively low cumulative alpha dose for (AmPO4)-Am-241.