New ethylenediphosphonates of molybdenum, A2[Mo 2O5(O3PCH2CH2PO 3)] (A = NH4 (1), Tl (2), Cs (3), Rb (4)), and K(H 3O)[Mo2O5(O3PCH2CH 2PO3)] (5), have been synthesized by a hydrothermal method and structurally characterized by X-ray diffraction, spectroscopic, and thermal studies. These compounds consist of pillared anionic layers [Mo 2O5(O3PCH2CH2PO 3)]2-, with A+, K+, and H 3O+ ions in the interlayer region as well as in the cavities within the anionic layers. Single-crystal X-ray structures of compounds 1 and 5 have been determined. They crystallize in the orthorhombic space group Cmca with Z = 8 and have the following unit cell parameters. For 1, a = 25.60(1), b = 10.016(4), and c = 9.635(3) Å and for 5, a = 25.63(1), b = 10.007(2), and c = 9.512(1) Å.