The crystal structures of dirubidium heptaoxodimolybdate, Rb(2)Mo(2)O(7), and dicaesium heptaoxodimolybdate, Cs(2)Mo(2)O(7), in the space groups Ama2 and P2(1)/c, respectively, have been determined for the first time by single-crystal X-ray diffraction. The structures represent two novel structure types of monovalent ion dimolybdates, A(2)Mo(2)O(7) (A = alkaline elements, NH(4), Ag or Tl). In the structure of Rb(2)Mo(2)O(7), Mo atoms are on a twofold axis, on a mirror plane and in a general position. One of the Rb atoms lies on a twofold axis, while three others are on mirror planes. Two O atoms attached to the Mo atom on a mirror plane are located on the same plane. Rubidium dimolybdate contains a new kind of infinite Mo-O chain formed from linked MoO(4) tetrahedra and MoO(6) octahedra alternating along the a axis, with two terminal MoO(4) tetrahedra sharing corners with each octahedron. The chains stack in the [001] direction to form channels of an approximately square section filled by ten-coordinate Rb ions. Seven- and eight-coordinate Rb atoms are located between chains connected by a c translation. In the structure of Cs(2)Mo(2)O(7), all atoms are in general positions. The MoO(6) octahedra share opposite corners to form separate infinite chains running along the c axis and strengthened by bridging MoO(4) tetrahedra. The same Mo-O polyhedral chain occurs in the structure of Na(2)Mo(2)O(7). Eight- to eleven-coordinate Cs atoms fill the space between the chains. The atomic arrangement of caesium dimolybdate has an orthorhombic pseudosymmetry that suggests a possible phase transition P2(1)/c-->Pbca at elevated temperatures.