The significant multireference character of the HSNO wavefunction is revealed from a comparison of its S-N bond properties calculated with CASSCF and Hartree-Fock methods. This is further supported by analysis of the CASSCF natural orbital occupation numbers and the configuration interaction expansion coefficients. Associating these results for HSNO with our description of S-nitrosothiols (RSNOs) as a combination of conventional R-S -N=O-, zwitterionic R-S+=N-O- , and RS-/NO+ ion-pair resonance structures, we demonstrate that the multireference character of the RSNO wavefunction arises from the ion-pair resonance structure. (c) 2008 Elsevier B. V. All rights reserved.