A novel method for measuring termination rate coefficients, kt, in free-radical polymerization is presented. A single laser pulse is used to instantaneously produce photoinitiator-derived radicals. During subsequent polymerization, radical concentration is monitored by time-resolved electron spin resonance (ESR) spectroscopy. The size of the free radicals, which exhibits a narrow distribution increases linearly with time t, which allows the chain-length dependence of kt to be deduced. The method will be illustrated using dodecyl methacrylate polymerization as an example.