A study is made of intrinsic acceptor site selectivity in the proton transfer from alkane radical cations to alkane molecules. The research is conducted by studying the relative degree of protonation at the various C-H bonds in undecane in irradiated CCl3F/undecane systems at cryogenic temperatures, It is observed that protonation occurs preferentially at the penultimate position, the difference between the other secondary C-H bonds being marginal; very little primary C-H protonation is observed under the experimental conditions.