An extensive quantum mechanical study of the potential energy surface for the isomerization and dissociation reactions of CHNS is reported. The calculations were performed using density functional theory and correlated ab initio methods and employing large aug-cc-pVTZ basis set. Nine CHNS isomers and eleven transition states linking them have been found on the singlet PES. Dissociation energies of the singlet most stable open chain CHNS isomers have been evaluated. On the triplet PES, eight mostly highly energetic CHNS isomers and five transition states for their interconversion and dissociation have been also found. Several available routes for isomerization and dissociation have been identified. A prediction has been made for the possible mechanism explaining the formation of the singlet HSCN and HSNC during UV photolysis of HNCS/Ar and HNCS/N-2 low-temperature matrixes observed recently by one of us.