The 1,3-dimethylcyclopentenyl carbenium ion (C7H11+) was reproducibly prepared on zeolite HZSM-5 using a pulse-quench reactor, and then each of a number of bases was coadsorbed into the catalyst channels to either compete with the cation for protonation or to possibly react with it as a nucleophile. For seven bases with proton affinities (PA) between 142 and 212.1 kcal/mol, there was no reaction with C7H11+ Coadsorption of smaller amounts of dimethylacetamide (PA = 217 kcal/mol) also produced no reaction, but with a higher loading, a proton was transferred from the carbenium ion to the base to leave 1,3-dimethylcyclopenta-1,3-diene in the zeolite as a neutral olefin. Deprotonation was the primary reaction with coadsorption of either pyridine (PA = 222 kcal/mol) or trimethylphosphine (PA = 229.2 kcal/mol). The estimated experimental deprotonation enthalpy for C7H11+, similar to 217 kcal/mol in the zeolite, is in excellent agreement with MP4/6-311G* gas-phase value of 215.6 kcal/mol. Coadsorption of either NH3 (PA = 204.0 kcal/mol) or PH3 (PA = 188 kcal/mol) does not deprotonate the carbenium ion, but these species do react as nucleophiles to form onium ion derivatives of C7H11+ Analogous onium complexes with pyridine or trimethylphosphine formed in lower yields due to steric constraints in the zeolite channels. The essential experimental observations were all predicted and explained by density functional calculations (B3LYP/6-311G**) and extensions of our recently developed theory of carbenium ion stability in zeolites. In addition, we report theoretical geometries for several complexes which contain unusual C-H---X hydrogen bonds.