A method is presented for determination of reliable values of the equilibrium constant K-eq between propagating radicals (P-center dot) and intermediate radicals (INT center dot) in reversible addition-fragmentation chain transfer (RAFT) polymerization. The technique is based oil EPR measurement of the ratio of concentrations of the two radical species Under quasi-equilibrium conditions ill low RAFT agent contents. The method requires no calibration of the EPR Setup for absolute radical concentration measurement. [Data for butyl acrylate polymerization mediated by benzyl propyl trithiocarbonate are presented. The value Of K-eq at - 40 degrees C was determined to be (1.0 +/- 0.1) x 10(4) L mol(-1).