Both single crystals and pure polycrystalline samples of three new quaternary alkali metal molybdenum selenites, Na2Mo2O5(SeO3)(2), K2Mo2O5(SeO3)(2), and Rb2Mo3O7(SeO3)(3), have been synthesized through hydrothermal and solid-state reactions using A(2)CO(3) (A = Na, K, and Rb), MoO3, and SeO2 as reagents. The frameworks of all three materials consist of both families of second-order Jahn-Teller distortive cations, i.e., the d(0) cation (Mo6+) and the lone pair cation (Se4+). Although the extent of framework distortions and the resulting occupation sites of alkali metal cations are dissimilar, Na2Mo2O5(SeO3)(2) and K2Mo2O5(SeO3)(2) exhibit similar three-dimensional networks that are composed of highly asymmetric Mo2O11 dimers and SeO3 polyhedra. Rb2Mo3O7(SeO3)(3) reveals a two-dimensional structure that is built with Mo3O15 trimers and SeO3 intralayer linkers. Close structural examinations suggest that the structure-directing effect of alkali metal cations is significant in determining the framework distortions and the dimensions of the molybdenum selenites. UV-vis diffuse reflectance and infrared spectroscopy, thermogravimetric analyses, and ion-exchange reactions are reported, as are out-of-center distortion and dipole moment calculations.