The mechanisms of a number of Mukaiyama aldol and Sakurai allylation reactions catalyzed by the Lewis acids [Ti(Cp)(2)(OTf)(2)], Ph(3)COTf, and Ph(3)CClO(4) have been investigated. It is found that hydrolysis of the Lewis acid by trace amounts of water in the solvent can lead to the formation of acid. The acid then reacts with the silyl enol ether or allylic silane to generate Me(3)SiOTf or Me(3)SiClO(4), both of which are powerful catalysts for these reactions. Dehydration of the solvent or addition of a hindered base to quench the acid does not necessarily prevent the formation of these silyl catalysts. In the case of the [Ti(Cp)(2)(OTf)(2)] Lewis acid, Me(3)SiOTf is generated as a consequence of the mechanism, and it is shown that all of the catalysis proceeds by the Me(3)SiOTf species and that [Ti(Cp)(2)(OTf)(2)] acts only as an initiator for the production of Me(3)SiOTf. For the case of the presumed Ph(3)COTf catalyst, the Mukaiyama aldol reaction proceeds exclusively by the Me(3)SiOTf catalyst and Ph(3)COTf is neither an initiator nor a catalyst. It generates Me(3)SiOTf by hydrolysis. It is shown that the rate and stereoselectivity of Me(3)SiOTf catalysis depend on the concentration of triflate ions. The rate slows and the selectivity switches with an increase in triflate concentration. The rate of Sakurai catalysis of allylsilane with an acetal using Ph(3)CClO(4) can be quantitatively accounted for by invoking only Me(3)SiClO(4) catalysis. The Me(3)SiClO(4) can be generated by hydrolysis, but when this is suppressed, Me(3)SiClO(4) can be formed by allylation of Ph(3)CClO(4). It is suggested that many of the reported catalysts may only be agents for the production of the real catalysts, namely, Me(3)SiX species. The characteristics of Lewis acids which may obviate the intrusion of Me(3)SiX catalysis are outlined.