A series of isotypic rare earth phosphites (RE = Ce(III), Pr(III), Nd(III), Pu(III), or Am(III)) with the general formulas RE2(HPO3)(3)(H2O) along with a Pu(IV) phosphite, Pu[(HPO3)(2)(H2O)(2)], have been prepared hydrothermally via reactions of RECl3 with phosphorous acid. The structure of RE2(HPO3)(3)(H2O) features a facesharing interaction of eight- and nine-coordinate rare earth polyhedra. By use of the crystallographic data from the isotypic series along with data from previously reported isotypic series, the ionic radii for higher coordinate Pu(III) and Am(III) were calculated. The Pu-VIII(III) radius was calculated as 1.112 +/- 0.004 angstrom, and the Pu-IX(III) radius was calculated to be 1.165 +/- 0.002 angstrom. The Am-VIII(III) radius was calculated as 1.108 +/- 0.004 angstrom, and the (IX)Arn(III) radius was calculated as 1.162 +/- 0.002 angstrom.