The scope of the ESR technique has been expanded to enable direct measurement of the individual propagating radical concentrations in a binary free radical copolymerization. The ESR spectra observed during the copolymerization of the deuterated monomers styrene-d(5) and cyclohexyl methaerylate-d(5) in benzene at 60 degreesC could be accounted for as the superimposed spectra of the homopolymerization systems, the outermost regions of the spectrum unambiguously assigned to macroradicals with styrene-d(5) as the terminal unit. The penultimate model with an implicit penultimate unit effect (PUE) was fitted to the ratio of the propagating radical concentrations (A(12)) and the copolymer composition, resulting in the radical reactivity ratios s(1) = k(211)/k(111) = 0.45 and s(2) k(122)/k(222) = 0.50. Simultaneous fitting of A(12) and the copolymerization propagation rate coefficient ((k) over bar)(p) resulted in lower s values (s(1) = 0.08, s(2) = 0.10). Optimization procedures employing the penultimate model with an explicit PUE and the entire data set (copolymer composition, A(12) and (k) over bar (p)) indicated that the total sum of residuals could be further decreased compared with when the restriction of an implicit PUE was imposed on the model, but this was at the expense of the quality of the prediction of copolymer composition, which then became unacceptable.