A detailed ab initio study of the structures and energetics of the persulfoxides and thiadioxiranes derived from sulfenic acid derivatives (RSX) is reported. The persulfoxides adopt structures in which the O-O bond bisects the RSX angle while the thiadioxiranes prefer a distorted trigonal bypyramidal geometry. The thiadioxiranes are more stable in every case than their persulfoxide isomer. The exothermicities of the interconversions of the persulfoxide to the thiadioxirane increase in the substituent (X) order CH3 < NH2 ( Cl < OCH3 < SCH3 ( F from a low of 3 kcal/mol to a high of 31 kcal/mol. The activation barriers, on the other hand, decrease in the substituent order Cl > CH3 approximate to NH2, OCH3 approximate to SCH3 > F from 27 to 10 kcal/mol. Only those persulfoxides which do not have a hydrogen on a heteroatom X exist in well-defined minima on the potential energy surface. Attempted minimization with tight convergence criteria of persulfoxides with heteroatom X-H bonds resulted in collapse via ene-like reactions to give hydroperoxy sulfonium ylides. In the case where X-H is N-H, the resulting hydroperoxysulfonium ylide (iminopersulfinic acid) adopts a hydrogen-bonded structure reminiscent of peracids. Experimental evidence for the formation of these new peroxides was obtained by photooxidations of N-methyl-, N-n-butyl-, and N-tert-butylbenzenesulfenamides.