Sulfinyl cations [R-S+=O (R = CH3, Ph, Cl, CH 3O and C2H5O)] have been demonstrated by MO calculations in conjunction with pentaquadrupole multidimensional (2D and 3D) MS2 and MS3 mass spectrometric experiments to be stable and easily accessible gas phase species, and their dissociation and ion/molecule chemistry have been studied. Potential energy surface diagrams indicate that the sulfoxides (CH3)2S=O, Ph2S=O, Cl 2S=O, (CH3O)2S=O and (C2H 5O)2S=O do not undergo rearrangement upon dissociative ionization, yielding the corresponding sulfinyl cations as primary fragments. Ph(CH3)S=O+., on the other hand, is predicted to isomerize to CH3-S-O-Ph+. via a four-membered ring transition state, yielding upon further CH3 . loss the isomeric ion S=O+...