The reaction of [(Me3Si)(2)N](2)S with equimolar amounts of SCl2 and S2Cl2 produces S4N2 in a good yield. The reaction of [(Me3Si)(2)N](2)S with a 3:1:1 mixture of S2Cl2, Se2Cl2 and SeCl4 yields a dark brown-red insoluble material that was inferred to be mainly SSeSMSN on the basis of the elemental analysis, mass spectroscopy, vibrational analysis, and NMR spectroscopy. Attempts to prepare selenium-rich species resulted in the formation of elemental selenium or Se3N2Cl2. The experimental work was supported by ab initio MO calculations which establish the structural and stability relationships of the different members of the series 1,3-SexS4-xN2 (x = 0-4). Full geometry optimization was carried out for each molecular species using the polarized split-valence MIDI-4* basis sets. The effects of electron correlation were taken into account involving the second-order Moler-Plessett perturbation theory. Each molecule was found to lie in an approximate half-chair conformation that is well established for 1,3-S4N2 (i.e., interacting planar NEN and EEE fragments; E = S, Se). The bond parameters agree well with experimental information where available. Whereas the lengths of the bonds in the NEEEN fragment approach those of the single bonds, the bonds in the NEN fragment show marked double bond character. The stabilities of the molecules decrease expectedly with increasing selenium content as judged by the total binding energy at the MP2 level of theory. Within a given chemical composition, isomers containing a N=Se=N unit lie higher in energy than those containing a N=S=N unit. These results may explain why selenium-rich SexS4-xN2 molecules have not been isolated.