The layered dichalcogenides1 MoS2 and MoSe2 are of special interest as lubricants, 2 catalysts, and catalyst supports for hydrodesulfurization reactions, 3 precursors of intercalation superconductors, 4 and possible electrodes for photoelectrolysis.5 In the pure, stoichiometric state they are moderate-gap semiconductors (1.75 eV for MoS2 and 1.4 eV for MoSe2), 5 presumably because the characteristic trigonal splitting of the d orbitals leads to filled d subbands. As part of a larger study on how small changes in chemical parameters affect superconductivity, we have had occasion to prepare the full solid solution series MoS2-xSex (0 < × < 2). The end compounds form both 2H and 3R polytypes. In each case, molybdenum atoms occupy trigonal-prismatic sites within the layers. In addition, in the system MoS2-xSex, a continuous series of solid solutions can be prepared, all having the same 2H two-layer hexagonal structure (space group P63/mmc). We report in this communication the evolution of lattice parameters of the mixed-anion system.