Rates of carbon protonation to give carbocation products were measured in concentrated perchloric acid solutions for cyclohexene, propyne, 1-hexyne, and their 1-trimethylsilyl-substituted analogs. The trimethylsilyl substituent accelerated the reaction markedly and provided the following beta-silyl carbocation stabilizing effects : delta Delta G double dagger = 5.7 kcal/mol(-1) for the cyclohexyl system and delta Delta G double dagger = 6.5 kcal mol(-1) for both acetylenic systems. These effects are substantially greater than delta Delta G double dagger = 2.9 and 3.4 kcal mol(-1) found previously for the protonation of ethyl vinyl ether and phenylacetylene, which suggests that the silyl effects in these previous systems were attenuated by additional carbocation stabilization provided through their ethoxy and phenyl groups. The present effects, on the other hand, fall far short of delta Delta G double dagger = 16-17 kcal mol(-1) found for conformationally optimum systems. The influence of conformation on the magnitude of beta-silyl effects and how this impinges on the presently studied systems is discussed.