Aryldiazenyl radicals were formed at −104–−116°C by photoirradiation of cyclopropane solutions of 1-aryl-2-(arylthio)diazenes and of l-aryl-3,3-dimethyltriazenes in the presence or absence of olefins, and also by irradiation of cyclopropane solutions of substituted 1,2,3-benzothiadiazoles in the presence of di-t-butyl peroxide. The ESR spectra were analyzed and the hfs constants were assigned by reference to the spectra of methyl, 2H, and 15N-labelled aryldiazenyl radicals and to the results of CINDO calculations. Aryldiazenyl radicals are relatively persistent and the –N=N group rotates about the C–N2 bond axis. The preferred conformations of the radicals were considered.