Tantalum hydrogen phosphate, beta-TaH(PO4)(2), has a three-dimensional structure that is stable to remarkably high temperature (similar to 600 degrees C) presumably due to the presence of strong hydrogen bonds. Impedance measurements indicate a low conductivity, 2.0 x 10(-6) S/cm at 200 degrees C in 5% H-2. In further studies aimed at enhancing the conductivity by aliovalent doping, we have investigated systematically the synthesis of compounds in the TaH(PO4)(2)-W2P2O11 system at 380 degrees C. As a result, a new phase, Ta-2(WO2)(0.87)H-0.26(PO4)(4), was identified and subsequently the molybdenum analog Ta-2(MoO2)(PO4)(4) was also prepared. The structures were determined by single crystal X-ray diffraction techniques. The structures of Ta-2(WO2)(0.87)H-0.26(PO4)(4) and Ta-2(MoO2)(PO4)(4) can be formally derived from the structure of beta-TaH(PO4)(2) by the replacement of two P-OH protons with an MO22+ (M = Mo and W) group together with a change in the orientation of some phosphate tetrahedra. (C) 2008 Elsevier Ltd. All rights reserved.