This investigation is concerned with the characterization of seleno-sulfide-halogen model systems, the isomerization processes, and the dissociation into diatomic fragment channels on the [H, S, Se, Cl] potential energy surface. Structural, energetic, and vibrational data were obtained at the CCSD(T) and MP2 levels of theory with the series of correlation consistent basis sets and extrapolated to the complete basis set (CBS) limit. For the frequencies, additional computations were performed to include the contribution of anharmonic effects, and for the determination of the heats of formation, important corrections incorporating core-valence correlation effects and relativistic effects (scalar and spin-orbit) were taken into account. CCSD(T)/CBS relative stability (kcal mol−1) follows the order: HSSeCl (0.0), HSeSCl (8.80), SSeHCl (23.52), and SeSHCl (25.87). The cis-rotational barrier for the two lowest isomers is practically identical (10.14 and 10.09 kcal mol−1), whereas for the trans barrier, we obtained 9.25 (HSSeCl) and 8.45 (HSeSCl) kcal mol−1. Dissociation of HSSeCl (HSeSCl) into HS (HSe) + SeCl (SCl) requires 59.70 (56.30) kcal mol−1. For the most stable isomer, we predict a value of the heat of formation at 298.15 K of 2.53 kcal mol−1. One of the outcomes of this research is that the MP2 results are consistent with those of CCSD(T). The MP2 method turns out to be a reliable alternative for a first exploration of larger catenated species, although it accounts for a lesser fraction of correlation effects. © 2012 Wiley Periodicals, Inc.