The gas-phase reactivity, in terms of deuteration versus alkylation, of radiolytically formed ethyl cations C2D5+ toward N-methylpyrrole and thiophene has been studied in CH4-C2D6 systems at 760 and 1520 Torr and in the presence of variable concentrations of triethylamine (TEA) with the aim of obtaining quantitative data on their competitive behavior. The substrate intramolecular selectivity has also been determined in both reaction channels. The gas-phase behavior of ethyl cations under high-pressure conditions demonstrates their predominant Bronsted versus Lewis acid reactivity toward the selected heteroaromatics, whose branching ratios, at a total pressure of 760 Torr, are 2.34 for N-methylpyrrole and 2.08 for thiophene. Intramolecular selectivity results indicate a substrate alkylation beta:alpha 58:42 for N-methylpyrrole and beta:alpha = 46:54 for thiophene. The substrate deuteration, determined by H-2 NMR, is beta:alpha 50:50 for N-methylpyrrole and beta:alpha = 36:64 for thiophene.