The three dimensional CASSCF-MRCI potential energy functions have been calculated for the X(2)A' and A(2)A" electronic states of HNS+ and for the X(2)A' state of the HSN+ isomer. In HN+, the two lowest states form a linear-bent Renner-Teller pair, the MRCI barrier to linearity for the X-state has been calculated to be 1876 cm(-1). Both isomers possess a conical intersection resulting from the crossing between the (2)Sigma(+) and (2)Pi states along the colinear H(S-2) + NS+(X(1)Sigma(+)) dissociation path. The minima of both isomers are found to be separated by a large barrier of 18,042 cm(-1) (CASSCF), the HNS+ isomer is calculated to be more stable than HSN+ by 12,202 cm(-1) (34.9 kca/mol) (RCCSD(T)). The proton affinity of the NS radical (NS (X(2)Pi + H+) has been calculated to be 7.59 eV (RCCSD(T)), the dissociation energy of HNS+ --> H(S-2) + NS+ (X(1)Sigma(+)) to be 2.86 eV (RCCSD(T)). Due to several low-lying electronic states of NS+, the protonation and the exothermic charge transfer reactions H+ --> NS --> + NS+ are found to proceed via very complex reaction paths including vibronic and spin-orbit couplings. The results of Renner-Teller variational calculations of the rovibronic levels for the electronic ground state of HNS+ are presented. The onset of this coupling is calculated already in the first bending overtone of the X-state. Also the vibrational levels obtained variationally for the X(2)A' state of the HSN+ isomer are given. (C) 2002 Elsevier Science B.V. All rights reserved.