Two new manganese(III) phenyl phosphonates, Mn(HO3PC6H5)(O3PC6H5). H2O and Mn(HO3PC6H5)(O3PC6H5), were synthesized. The crystal structure of Mn(HO3PC6H5)(O3PC6H5) was solved ab initio from powder X-ray diffraction data. This compound crystallizes in the triclinic system, a = 10.2149(5), b = 15.0028(8), c = 5.0324(3) Angstrom, alpha = 95.563(4)degrees, beta = 116.858(4)degrees, gamma = 95.684(5)degrees, V = 675.87(6) Angstrom(3), space group P (1) over bar, and Z = 2. The final agreement factors were R-WP = 12.8%, R-P = 9.1%, and R-F = 3.2%. There are 22 non-hydrogen atoms in the asymmetric part of the unit cell, and the positional parameters were refined with the help-of soft constraints. The octahedral manganese coordination spheres are distorted due to the Jahn-Teller effect. The structure of this organic-inorganic compound is layered. The thermal behavior of Mn(HO3PC6H5)(O3PC6H5). H2O was studied and its thermal decomposition product was identified.