We have investigated rarely observed Se-77 J-couplings (spin-spin couplings) in the mixed-metal face-capped octahedral clusters [Re5OsSe8(CN)(6)](3-) and [Re4Os2Se8(CN)(6)](2-) at natural abundance. To the best of our knowledge, these are the first observations of Se-Se spin-spin interactions between mu(3)-Se sites, important for stereochemical assignments in hexarhenium analogues, Chevrel phase materials, and similar cluster materials. NMR techniques such as COSY, INADEQUATE, and 2D J-resolved spectroscopy have been used in conjunction to study these interactions. The two isomers (cis and trans) of [Re4Os2Se8(CN)(6)](2-) were distinguishable, and selective isotopic labeling of [Re5OsSe8(CN)(6)](3-) with (CN)-C-13 ligands enabled resonances to be assigned by observing the (2)J (Se-M-C) couplings. For [Re5OsSe8(CN)(6)](3-), two different (2)J (Se-M-Se) couplings were measurable on a single cluster, and these are related to one another through spin-spin interactions across a face diagonal or along an edge of the cube of inner selenium ligands. A rigorous analysis based on combinatorial math has been invoked to assign the couplings on the basis of the probability of multiple-spin interactions. The face diagonal association is found to result in a J-coupling interaction larger in magnitude than that from coupling along an edge of the cubeinformation critical for making stereochemical assignments of selenium sites.