Activation barriers (Delta H-Me(double dagger)) for adding methyl radicals to ions of the general formula CH3CR=OCH3+ have been measured by looking at the threshold energies for the reverse reaction, dissociative photoionization of ethers of the general formula RC(CH3)(2)OCH3. Dissociation by loss of a methyl radical has more favorable thermochemistry than loss of R center dot yet the onset of R center dot loss occurs at lower energies than loss of CH3center dot. In other words, the more endothermic dissociation exhibits a lower appearance energy. Contrathermodynamic ordering of appearance energies is observed for R = Et, nPr, iPr, tBu, and neopentyl. The sum of the appearance energy difference, Delta AE, and the thermochemical difference (Delta Delta H, calculated using G3 theory) gives a lower bound for the barrier for adding methyl radical to CH3CR=OCH3+. More specifically, the difference between that activation barrier and the one for adding R-center dot to (CH3)(2)C=OCH3+, Delta H-Me(double dagger) - Delta H-R(double dagger), equals Delta AE + Delta Delta H and has values in the range 20-24 kJ mol(-1) for the homologous series investigated. There is no systematic trend with the steric bulk of R, and available evidence suggests that Delta HRdouble dagger does not have a value > 5 U mol(-1). The difference in barrier heights, AH(Me)(double dagger) - AH(iPr)(double dagger) for CH(3)center dot Plus iPrC(CH3)=OX+ vs iPr center dot + (CH3)(2)C= OX+, has the same value, regardless of whether X = H or CH3. Mixing of higher energy electronic configurations provides a qualitative theoretical explanation for some (but not all) observed trends in barrier heights.