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A combined experimental and theoretical approach has been employed to establish the basicity and proton affinity of SiF4 and the structure of SiF4H 1 . The kinetics and energetics for the transfer of a proton between SiF4 ,N 2, and Xe have been explored experimentally in helium at 0.35 6 0.02 torr and 297 6 3 K with a selected-ion flow tube apparatus. The results of equilibrium constant measurements are reported that provide a basicity and proton affinity for SiF4 at 297 6 3 K of 111.4 6 1.0 and 117.7 6 1.2 kcal mol 21 , respectively. These values are more than 2.5 kcal mol 21 lower than currently recommended values. The basicity order was determined to be GB(Xe) . GB(SiF4) . GB(N2), while the proton-affinity order was shown to be PA(Xe) . PA(N2) . PA (SiF4). Ab initio molecular orbital computations at MP4SDTQ(fc)/ 6-31111G(3df,3pd) using geometries from B3LYP/6-311G(d,p) indicate a value for PA(SiF4) 5 118.7 kcal mol 21 that is in good agreement with experiment. Also, the most stable structure of SiF4H 1 is shown to correspond to a core SiF3 cation solvated by HF with a binding energy of 43.9 kcal mol 21 . Support for this structure is found in separate SIFT collision induced dissociation (CID) measurements that indicate exclusive loss of HF. (J Am Soc Mass