We present evidence that the reactions of ethenesulfonyl chloride (1) andtrans-1-propene-1-sulfonyl chloride (3) with water in the presence of pyridine, trimethylamine, and a number of other tertiary amines proceed primarily by way of an initial vinylogous substitution reaction to form the cationic sulfene,, which subsequently reacts with water either by addition (and deprotonation) to form the betaine, or by vinylogous substitution (and deprotonation) to give the alkenesulfonate anion,(R = H or CH3). Formation of the latter represents the first well-supported example of vinylogous nucleophilic catalysis. These conclusions are drawn from kinetic and product composition observations, including (a) α-monodeuteration in the betaine and lack of deuteration of the ethenesulfonatefrom the reaction in D2O, (b) rate lowerings of up to 2000-fold for 2- (and 6-) substituted pyridines from those expected from Brønsted-type relationships shown by "unhindered" pyridine bases, (c) lack of a kinetic solvent isotope effect in the reaction of1with 3-cyanopyridine, (d) a lower rate of reaction of3vs.1not directly correlated with product composition, and (e) formation of similar product mixtures from either1or Pyr+CH2CH2SO2Cl Cl−(18a) with aqueous pyridine. For the initial formation of the sulfene,, the available evidence does not distinguish between a two-step mechanism via an intermediate zwitterion and a closely related concerted reaction, but for the further reaction of the sulfene a process involving a zwitterionic intermediate common to both products is favoured. For the reaction of1or3in the absence of tertiary amines evidence is presented for a direct displacement on sulfur mechanism leading to the alkenesulfonate anion, plus a small proportion (up to 15%) of formation of the 2-hydroxy-1-alkanesulfonate anion by way of the sulfene HOCHRCH=SO2(R = H or CH3).