Interactions between reactive free radicals (r) with stable mononitroxyl radicals (N) and bisnitroxyl radicals (N-X-N) were studied by time- resolved electron paramagnetic resonance (TR-EPR). Reactive spin- polarized free radicals ( r(#)), with non- Boltzmann population of spin states were produced by laser flash photolysis of benzil dimethyl monoketal or of (2,4,6- trimethylbenzoyl) diphenyl phosphine oxide ( the superscript # symbol indicates electron spin polarization). Both isotopically symmetric nitroxyl biradicals (N-14-X-N-14) and isotopically asymmetric nitroxyl biradicals, with one nitroxyl bearing N-15 and the other nitroxyl bearing N-14 (N-14-X-N-15), were employed as probes of the spin exchange and chemical interactions between r and the nitroxyl biradicals. The interaction of r(#) with the asymmetric ortho-nitroxyl biradical (N-14-O-N-15), which exists in a condition of strong spin exchange, proved to be particularly informative. In this case, spin polarized (N-14-O-N-15)(#) ( product of spin exchange with r(#)) and two polarized monoradicals (r(14)N-O-N-15)(#) and (N-14-O-(15)Nr)(#) ( products of chemical reaction with r(#)) were observed. The latter three species possess three distinct TR- EPR spectra with different line splittings. The relative cross sections for spin exchange ( R-ex) and chemical reaction ( R-rxn) were achieved through computer simulation of the TR-EPR spectra. The cross section for spin exchange, R-ex, between r(#) and ( N-X-N) biradical is estimated to be 4-6 times larger than the cross section of chemical reaction, R-rxn, between r(#) and (N-X-N). The paranitroxyl biradical (N-14-P-N-15) exists in weak spin exchange, and behaves as an equimolar mixture of N-14 and N-15 mononitroxyls.