A new binary indium phosphate monohydrate, InPO4. H2O was synthesized under hydrothermal conditions. Its crystal structure was determined using single-crystal X-ray diffraction methods. InPO4. H2O crystallizes in the triclinic system, space group P (1) over bar, with a = 5.4342(6) Angstrom, b = 5.5508(4) Angstrom, c = 6.5446(5) Angstrom, alpha = 97.593(6)degrees, beta = 94.558(6)degrees, gamma = 107.565(6)degrees, and Z = 2. The 3-D framework is built from (In2O8(OH2)(2)) dimers connected through (PO4) tetrahedra. Water molecules directly bond to indium. Thermogravimetric studies indicate that InPO4. H2O loses one water molecule in the temperature range between 370 and 480 degrees C to yield the previously reported anhydrous InPO4. The loss of water corresponds to condensation of the (In2O8(OH2)(2)) dimers into linear chains of edge-sharing (InO6) octahedra. InPO4. H2O represents the missing link between InPO4 and InPO4. 2H(2)O. The structural relationships between InPO4. 2H(2)O, InPO4. H2O, InPO4, and other M-III(PO4) 2H(2)O are discussed.