Examination of literature values for the thermolysis of neopentylcobalamin (NpCbl) in aqueous solution shows significant variation in the observed activation parameters. Re-evaluation of the reaction conditions and the techniques employed reveals that this variation is due to errors in temperature maintenance and measurement. Thus, the thermolysis of the carbon-cobalt bond of NpCbl in aqueous solution has been reexamined (T = 14.8-39.6 degrees C) taking particular care to carefully maintain and measure the temperature. The resulting observed activation parameters, Delta H-obs(double dagger) = 26.5 +/- 0.1 kcal mol(-1) and Delta S-obs(double dagger) = 12.5 +/- 0.3 cal mol(-1) K-1, are larger than values previously reported which we believe in part to be attributable to this small but significant systematic error. The thermodynamic parameters for the base-off/base-on equilibrium of NpCbl have been determined by an NMR method which is also shown to be susceptible to a potential systematic error in temperature measurement. The thermodynamic parameters for the base-off/on equilibrium were found to be Delta H-meas = -4.1 +/- 0.1 kcal mol(-1) and Delta S-meas = -12.5 +/- 0.3 cal mol(-1) K-1. Using these values, the observed rate constant for NpCbl thermolysis can be corrected for the substantial amount of base-off species present in neutral aqueous solution. The corrected activation parameters for the base-on species are Delta H-on(double dagger) = 28.3 +/- 0.2 kcal mol(-1) and Delta S-on(double dagger) = 19.3 +/- 0.6 cal mol(-1) K-1. Methods for data analysis and the effects or errors in temperature measurement on the final results are discussed in detail.