Propagating radicals of o-, m-, and p-substituted styrenes were detected using ESR spectroscopy during their bulk or benzene solution polymerizations. The hyperfine coupling constants for the protons of the monomeric unit carrying an unpaired electron was determined. Similar to the benzyl-type small radicals, an increase in the resonance stabilization of the polymer radical only slightly decreases the coupling constants for the alpha-and beta-protons (a(alpha) and a(beta), respectively). Plots of a(alpha) and a(beta) of the polymer radicals versus sigma(alpha)(.) which has been defined as a substituent constant based on sigma(alpha)(.) of the nuclear substituted benzyl radicals gave linear relationships. A substituent of which ob, value is positively large indicating low spin density at benzyl positions resulted in a decreases in a, and as. Plots of the a, and as for the polymer radical versus the Q valuefor the substituted styrene showed that an increase in the Q value brings about a decrease in the coupling constants. The hyperfine coupling constants for the m-, o-, and p-protons were also evaluated.