Standard enthalpies of formation of several thionitroso (XNS) and thiazyl (XSN) isomers, with X = H, F, Cl, Br, OH, SH, NH2, CH3, CF3, and SiF3, were determined using coupled cluster (CC) theory with Dunning's correlation consistent basis sets cc-pVXZ and aug-cc-pV(X+d)Z, G3 and CBS-QB3 model chemistries, as well as the B3LYP DFT method. The results support the idea that the electronegativity of the bonding atom in the substituent is correlated, albeit not perfectly, with the relative stability of the XNS over the XSN isomer. A detailed study was performed on the parent isomers HNS and HSN. They exhibit a singlet (1)A' ground state (as all the other molecules) at 5.4 and 9.4 kcal/mol below their first excited (3)A" state, respectively (CCSD(T)/CBS calculations). The enthalpies of formation of the isomers at the CCSD(T)/CBS limit, including core valence correlation, and spin-orbit splitting, are Delta(f)Hdegrees(298)(HNS) = 55.3 +/- 1 kcal/mol and Delta(f)Hdegrees(298)(HSN) = 75.4 +/- 1 kcal/mol. The activation energy at 0 K for the HNS --> HSN isomerization was determined as 63.0 and 64.2 kcal/mol at the CCSD(T)/CBS and B3LYP/6-311+G(3df,2p) levels of theory, respectively. Additional calculations were performed for the cationic isomeric pairs XNS+/XSN+ with the general result that ionization increases the relative stability of the thionitroso isomer with respect to the thiazyl isomer. In the cases of CISN and BrSN, for which the thiazyl isomer is the most stable neutral species, the thionitroso isomer is more stable for the cations. The average deviation of the B3LYP/6-311+G(3df,2p) ionization potentials with respect to the G3 model chemistry is only 0.1 eV.