A combined experimental and theoretical approach has been employed to establish the basicity and proton affinity of SiF 4 and the structure of SiF 4 H + . The kinetics and energetics for the transfer of a proton between SiF 4 , N 2 , and Xe have been explored experimentally in helium at 0.35 +/- 0.02 torr and 297 +/- 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 SiF 4 at 297 +/- 3 K of 111.4 +/- 1.0 and 117.7 +/- 1.2 kcal mol - 1 , respectively. These values are more than 2.5 kcal mol - 1 lower than currently recommended values. The basicity order was determined to be GB(Xe) > GB(SiF 4 ) > GB(N 2 ), while the proton-affinity order was shown to be PA(Xe) > PA(N 2 ) > PA (SiF 4 ). Ab initio molecular orbital computations at MP4SDTQ(fc)/6-311++G(3df,3pd) using geometries from B3LYP/6-31+G(d,p) indicate a value for PA(SiF 4 ) = 118.7 kcal mol - 1 that is in good agreement with experiment. Also, the most stable structure of SiF 4 H + is shown to correspond to a core SiF 3 + cation solvated by HF with a binding energy of 43.9 kcal mol - 1 . Support for this structure is found in separate SIFT collision induced dissociation (CID) measurements that indicate exclusive loss of HF.