The stabilization afforded a vinyl cation by a beta-(CH3)(3)Si substituent has been determined by measuring in a high-pressure mass spectrometer the thermodynamic data for the association of three alkynes (RCCR’) with (CH3Si+ and with the proton. The measured -Delta H degrees (kcal mol(-1)) and -Delta S degrees (in parentheses, cal K-1 mol(-1)) values for the reaction (CH3Si+ + RCCR’ reversible arrow (CH3)(3)Si . C(R)CR’(+) are as follows : 1-hexyne (R = H, R’ = n-C4H9) 25.9 +/- 1.5 (19.1 +/- 0.2), 2-hexyne (R = CH3, R’ = n-C3H7) 28.8 +/- 1.4 (25.5 +/- 0.3), and phenylacetylene (R = H, R’ = C6H5) 28.2 +/- 2.8 (16.5 +/- 0.4). By comparison the values for 1-hexene which forms an alkyl cation are 38.2 +/- 0.5 kcal mol(-1) (48.2 +/- 0.1 cal K-1 mol(-1)). The deduced stabilizations (A) for all the substituents (R, R’ and (CH3)(3)Si) obtained from the isodesmic reaction (CH3)(3)Si . C(R)CR’(+) + CH2CH2 --> (CH3)(3)Si . C(R)C(H)R’ + CH2CH+ are (kcal mol(-1)) as follows : 1-hexyne 55, 2-hexyne 58, and phenylacetylene 58. The deduced stabilization for the (CH3Si+ adduct of l-hexene relative to the ethyl cation is 60 kcal mol(-1). The measured proton affinities are (kcal mol(-1)) as follows : 1-hexyne 194.5 +/- 0.5, 2-hexyne 195.8 +/- 0.2, phenylacetylene 198.6 +/- 0.2, and 1-hexene 194.0 +/- 0.5. The stabilizations (B) due to R and R’ in the vinyl cations RC(H)=CR’(+) produced by protonaton are calculated from the isodesmic reactions RC(H)CR’(+) + CH2CH2 --> RC(H)C(H)R’ + CH2CH+ and are (kcal mol(-1)) as follows : 1-hexyne 44, 2-hexyne 46, and phenylacetylene 50. The comparable value for the alkyl cation from the protonation of 1-hexene is 34 kcal mol(-1). The stabilizations of the vinyl cations RC(H)=CR’(+) due to the presence of a beta-(CH3)(3)Si (A - B) are (kcal mol(-1)) as follows : 1-hexyne 11, 2-hexyne 12, and phenylacetylene 9. For the alkyl cation formed from 1-hexene, the value is 26 kcal mol(-1). The stabilization of a vinyl cation by an alpha-alkyl or alpha-aryl substituent is substantially greater than that afforded by the same substituent in an alkyl cation. The total stabilization afforded by both an a-alkyl or a-aryl substituent and a beta-(CH3)(3)Si substituent appears to be approximately the same in both alkyl and vinyl cations and hence the beta-silicon effect is considerably smaller for the vinyl cation.