The conversion of several stereoisomeric osymercurials to allcenes (deosymercuration) has been studied with hydrogen chloride in anhydrous and near-anhydrous methanol. In each instance the addition of water decreases the second-order rate. In benzene the addition of water to the anhydrous system first accelerates and then decreases the reaction rate. These results have been interpreted in support of molecular rather than ionic participation by hydrogen chloride. Since molecular participation requires the TT arrangement of alkoxy and chloromercuri groups for easy reaction it follows that oxymercuration also follows a molecular and not a "rnercurinium ion" mechanism in the normal reaction.The decomposition of 2-oxyorganomercuric salts to mercuric salts and alltenes has been called deoxymercuration (11). I t is a much milder reaction than is the scission of simple alkylmercuric salts, some of which resist prolo~lged heating with strong acids. This difference must be due to the vici~lal oxygen linltage. Several opinions have been expressed about the mallller in which this oxygen assists the scission of the carbon-mercury link. I t will be seen that these opi~lions are closely related to the ideas of oxymercurial formation. For this reason a decision about the manner in which deoxymercuration occurs ought to specify the mechanism of oxymercur a t' lon.According to Whitmore (20) deoxymercuration is induced by formation of an oxonium salt, I, which then decomposes into an alcohol, an alkene, and a mercuric rnonocation (where X is an acid substituent bound covalently).According to this mechanism the rate of deoxymercuration ought to be dependent on the strength of the acid used in the reaction. This dependence on acid strength has been confirmed by experiment (1 1).According to another, more recent, opi~lion the reaction is of the "E2 elimination" type (22) in which an anion is presumed to coordinate with the mercurial (11) to force the -8