RRKM theory is combined with a steady-state master equation approach to calculate unimolecular rate constants as a function of temperature and pressure for dissociation of (CH3)2Zn, (C2H5)2Zn, (CH3)2Cd, and (CH3)2Hg. The dissociation energy of the first metal-alkyl bond is treated as an adjustable parameter to fit earlier pyrolysis data. The master equation explicitly accounts for the pressure dependence of the measured rate constants and allows an accurate determination of the bond dissociation energies for the first and second metal-alkyl bonds of each compound. The second metal-alkyl bonds of the zinc and cadmium compounds are found to be stronger than previously believed, accounting for the observation of CH3Zn, C2H5Zn, and CH3Cd upon photodissociation of the corresponding dialkyls. © 1989.