Investigations into the kinetics of primary radicals produced in photochemically and thermally induced decomposition of peroxides of type R1C(O)O-OR2 are presented. The correlation of peroxide structure with decomposition rate and with initiator efficiency in radical polymerizations is discussed. Termination rate coefficients, kt, as a function of temperature, pressure, polymer content, and of chain length may be deduced from two types of time-resolved experiments in which, after applying an excimer laser pulse, either monomer conversion is measured via near-infrared spectroscopy or the decay in radical concentration is monitored via electron spin resonance.