An ab initio investigation of the gas-phase hydride transfer reaction RwH ] R`] [RwHwR]`] R`] RwH has been performed. Four examples of this reaction, with R \ methyl, ethyl, sec-propyl and tert-butyl fragments were considered. The potential-energy surfaces (PES) for all these cases found at the MP2 level have potential wells which correspond to the symmetric [RwHwR]`complexes. Such PES are di †erent from those for hydride transfer in liquid super acids and especially in heterogeneous zeolite catalysts. Indeed, solvation e †ects change the relative stability of carbenium ions R`and carbonium ions [RwHwR]`in liquid super acids. Therefore, the latter species correspond to small barriers on PES for hydride transfer, instead of potential wells in the gas phase. In zeolite catalysts, the interaction of carbenium ions R`with zeolite surfaces is much stronger and results in formation of covalent surface esters instead of ion pairs, whereas carbonium ions [RwHwR]`represent highenergy activated complexes.