Hydrothermal reactions of lanthanide(III) oxide, molybdenum oxide, and SeO2 at 230 degrees C lead to five new molybdenum-rich quaternary lanthanide selenites with two types of structures, namely, H(3)Ln(4)Mo(9.5)O(32)(SeO3)(4)(H2O)(2) (Ln = La, 1; Nd, 2) and Ln(2)Mo(3)O(10)(SeO3)(2)(H2O) (Ln = Eu, 3; Dy, 4; Er, 5). Compounds 1 and 2 feature a complicated three-dimensional (3D) architecture constructed by the intergrowth of infinite molybdenum selenite chains of [Mo4.75SeO19](5.5-) and one-dimensional (ID) lanthanide selenite chains. The structures of 3, 4, and 5 exhibit 3D network composed of ID [Mo3SeO13](4-) anionic chains connected by lanthanide selenite chains. The molybdenum selenite chain of [Mo4.75SeO19](5.5-) in 1 and 2 is composed of a pair of [Mo3SeO13](4-) chains as in 3, 4, and 5 interconnected by a [Mo1.75O8](5.5-) double-strand polymer via corner-sharing. The lanthanide selenite chains in both structures are similar in terms of coordination modes of selenite groups as well as the coordination environments of lanthanide(III) ions. Luminescent studies at both room temperature and 10 K indicate that compound 2 displays strong luminescence in the near-IR region and compound 3 exhibits red fluorescent emission bands with a luminescent lifetime of 0.57 ms. Magnetic properties of these compounds have been also investigated.