The isomerization of cyclohexylium to methylcyclopentylium is a model for a key step required in sterol and triterpene biosynthesis and is important in catalytic processes associated with ring-opening reactions in upgrading petroleum fractions. Using high-level, correlated wave function techniques based on QCISD, the mechanism for this isomerization was found to be very different from that first proposed more than 35 years ago. On the basis of our mechanism, a first-order rate constant expression was derived and used with complete basis set-extrapolated QCISD(T) energies to obtain E-a = 6.9 kcal/mol and A = 10(11.18) s(-1), in excellent agreement with values of 7.4 +/- 1 kcal/mol and A = 10(12) (+/-) (1.3) s(-1) measured in the gas phase. The B3LYP and MP2 methods, two commonly used computational approaches, were found to predict incorrect mechanisms and, in some cases, poor kinetic parameters. The PBE method, however, produced a reaction profile and kinetic parameters in reasonable agreement with those obtained with the complete basis set-extrapolated QCISD(T) method.