Crystals of bis(5,5-dimethyl-2-thioxo-1,3,2-dioxaphosphorinan-2-yl) disulfide (1a) grown from benzene are monoclinic, Cc, with a = 6.762(2) Angstrom, b = 21.856(4) Angstrom, c = 12.061(3) Angstrom, a = 90", beta = 93.49(2)degrees, gamma = 90 degrees, V = 1779.2(5) Angstrom(3), Z = 4, and D-c = 1.47 g cm(-3). Refinement using 1789 reflections for 201 variables gives R = 0.036. Crystals of bis(5,5-dimethyl-2-thioxo-1.3,2-dioxaphosphorinan-2-yl) diselenide (2b) grown from a mixture of benzene/hexane contain benzene which upon exposure to the atmosphere diffuses out over several weeks giving the diselenide 2a. Crystals of 2a are rhombohedral, R (3) over bar, in a hexagonal setting with a = b = 31.660(3) Angstrom, c = 9.702(1) Angstrom, V = 8422(38) Angstrom(3), Z = 18, and D-c = 1.732 g cm(-3). Refinement using 3369 reflections for 181 variables gives R = 0.054. C-13, P-31, and Se-77 high-resolution solid state NMR data are consistent with results obtained from single crystal diffraction studies and confirm that in both compounds one molecule per unique volume is an independent unit. Principal elements of the P-31 and Se-77 chemical shift tensors calculated from spinning sideband intensities are discussed in terms of the local geometry of phosphorus and selenium.