Tert-butoxy radicals, derived from the thermal decomposition of di-tert-butyl peroxalate at 60°C, react with methyl, ethyl, and n-butyl methacrylate by double-bond addition and by hydrogen abstraction from the α-methyl and ester alkyl functions. The proportion of each of these processes, as well as the fraction of tert-butoxy radicals that fragment to give methyl radicals, has been determined by analysis of the products that result on trapping the transient carbon-centered radicals with 1,1,3,3-tetramethylisoindoline-2-oxyl.