The rearrangements RCMe 2CH 2· → RCH 2CMe 2 (k r) (R = Ph, Me 3CC≡C, Me 3CC=O, and N=C) have been studied over a range of temperatures by product analyses with use of the common competing reaction RCMe 2CH 2· + CCl 4 → RCMe 2CH 2Cl + CCl 3· (k Cl). For R = H 2C=CH the rearrangement was so fast that only the rearranged chloride, RCH 2CMe 2Cl, was produced. All these rearrangements occur via a 3-membered cyclic intermediate radical (or transition state). Various considerations led to the following Arrhenius equation for chlorine abstraction: log (k Cl/M -1 s -1) = (8.14 ± 0.42) - (5.52 ±0.63)/θ, where θ = 2.3RT kcal/mol, and this equation is used to calculate Arrhenius parameters for migration of all but the H 2O=CH group. Comparison of these parameters with those already available from kinetic EPR measurements leads to a choice of preferred Arrhenius parameters for all five rearrangements. The cyano group had an unexpectedly low mobility while the pivaloyl group underwent a surprisingly rapid 1,2-shift. Migratory aptitudes increase along the series R = N≡C < Me 3CO≡C < Ph < Me 3CC=O < H 2C=CH, with k r at 25°C = 0.9, 93, 762, 1.7 × 105, and 1.0 × 10 7 s -1, respectively. The preferred pre-exponential factors all lie in the range 10 10.9-10 12.0 s -1, while the activation energies vary from 16.4 kcal/mol for R = N≡C to 5.7 kcal/mol for R = H 2C=CH. These results are discussed in relation to the results of thermochemical kinetic calculations and to earlier work on the 1,2-migration of unsaturated groups in radicals.