Deprotonation of thiirane, thiirane S-oxide, and thiirane S,S-dioxide was carried out in the gas phase, and the resulting anions were studied through a series of ion-molecule reactions. The cyclic conjugate bases of thiirane (1b) and thiirane S,S-dioxide (3a) are formed whereas a facile ring opening isomerization to vinyl sulfenate anion (2d, CH2=CHSO-) is observed in the thiirane S-oxide case. Vinyl thiolate, the rearrangement product of the conjugate base of thiirane, also is obtained. Proton affinities were determined for 1b, 2d, and 3a. High-level ab initio calculations were also carried out, and the results compared with those obtained from experiment. Orbital arguments are advanced to explain the relative barriers to ring opening for the three anions of interest.