Density functional theory (DFT) and ab initio (CBS-RAD) calculations have been used to investigate aseries of “radical clock” reactions. The calculated activation energies suggest that the barriers for theseradical rearrangements are determined almost exclusively by the enthalpy effect with no evidence ofsignificant polar effects. The ring-closure reactions to cyclopentylmethyl radical derivatives and the ringopening of cyclopropylmethyl radicals give different correlations between the calculated heat of reactionand barrier, but the two types of reaction are internally consistent.