Addition and fragmentation rate coefficients, k(ad) and k(beta), have been measured by individually tracing intermediate and propagating radical concentrations via highly time-resolved EPR spectroscopy after applying a laser single pulse. The method is illustrated for butyl acrylate polymerization at -40 degrees C with EDTCP (ethyl 2[1-diethoxyphosphoryl-2,2,2-trifluoroethoxythio carbonylsulfanyl] propionate) being the MADIX agent Analysis of the radical concentration vs. time profiles yields k(ad) = (2.5 +/- 0.1) x 10(4) L mol(-1) s(-1) and k(beta) = (2.3 +/- 0.3) x 10(3) s(-1). The associated K-eq = k(ad)/k(beta) value is in close agreement with K-eq deduced from an earlier EPR method which rests on measuring the ratio of intermediate and propagating radical concentrations during stationary polymerization. (C) 2010 Elsevier Ltd. All rights reserved.