The branching ratio, protonation versus alkylation, of radiolytically formed propyl cations toward N-methylpyrrole has been studied in C3D8 and in CH4-C3D8 systems at 760 and 1520 Torr and in the presence of gaseous bases such as trimethylamine (TMA) and triethylamine (TEA). Propane-d(8) used as source of C3D7+ cations allowed the Bronsted reactivity of the reagent cations and the relevant substrate intramolecular selectivity to be evaluated. The results obtained indicate a Bronsted versus Lewis reactivity ratio ranging between 0.90 in the C3D8 bulk gas and 0.55 in the CH4-C3D8 mixture : Intramolecular selectivity results show a beta:alpha = 6:9 substrate alkylation and a beta:alpha = 33:67 substrate deuteration.