Abstract N-Nitroso-p-chloro(36Cl)acetanilide has been decomposed in mixtures of benzene and mono-substituted benzene C6H5X (X=NO2, Cl, OCH3, and CH3), and the reaction products have been analyzed for 4-chlorobiphenyl and isomeric 4-ClC6H4C6H4X by the isotope dilution method. On the basis of these results, the partial rate factors for the p-chlorophenylation have been calculated to be, for nitrobenzene: o, 4.35; m, 0.61; p, 6.18; for chlorobenzene: o, 2.70; m, 0.87; p, 1.33; for anisole: o, 3.93; m, 0.94; p, 1.54 and for toluene: o, 2.97; m, 1.07; p, 1.32. The values of the partial rate factor for the meta position give a satisfactory Hammett plot with a slope of −0.27. This fact demonstrates that the p-chlorophenyl radical shows a measure of electrophilic character in the homolytic aromatic substitution.