Collisional activation (CA) and ion-molecule reaction experiments making use of a hybrid tandem mass spectrometer provide evidence for the dilute gas-phase existence of the nitroso sulfonium cation (H2SNO+). Ab initio molecular orbital calculations using the coupled-cluster method, CCSD(T), with both 6-31G(d,p) and 6-311 + + G(3df,2p) basis sets confirm that, while H2SNO+ is relatively stable with respect to fragmentation, it can best be regarded as a strong molecular complex between NO+ and SH2 rather than an onium cation. Other nitroso onium cations have also been generated by ionization of mixtures incorporating NO. and water, methanol, ammonia and alkane thiols and characterized by collisional activation and by neutralization-reionization (NR) mass spectrometry. (C) 1998 Elsevier Science B.V.