The gas-phase stability of bridgehead and rigid tertiary carbenium ions has been measured by equilibria for bromide transfer and equilibria for protonation of olefins by ICR techniques. Combination of the experimental data with the MM2-calculated enthalpies of formation of the neutrals gives the enthalpy change associated with the ionization process. The rate constants for solvolysis of the corresponding tertiary halides and sulfonate esters correlate linearly with these enthalpy changes. However, simple acyclic and monocyclic tertiary derivatives exhibit higher solvolytic reactivities than expected on the grounds of the thermodynamics of the reaction.