Crystal structure and ionic conductivity of ruthenium diphosphates, ARu(2)(P2O7)(2) A = Li, Na, and Ag, were investigated. The structure of the Ag compound was determined by single crystal X-ray diffraction techniques. It crystallized in the triclinic space group P-1 with a = 4.759(2) Angstrom, b = 6.843(2) Angstrom, c = 8.063(l) Angstrom, alpha = 90.44(2)degrees, beta = 92.80(2)degrees, gamma = 104.88(2)degrees, V = 253.4(1) Angstrom(3). The host structure of it was composed of RuO6 and P2O7 groups and formed tunnels running along the a-axis, in which Ag+ ions were situated. The ionic conductivities have been measured on pellets of the polycrystalline powders. The Li and Ag compounds showed the conductivities of 1.0 X 10(-4) and 3.5 x 10(-5) S cm(-1) at 150degreesC, respectively. Magnetic susceptibility measurement of the Ag compound showed that it did not obey the Curie-Weiss law and the effective magnetic moment decreased as temperature decreased due to the large spin-orbital coupling effect of Ru4+ ions. (C) 2003 Elsevier Science Ltd. All rights reserved.