Photochemical or thermal decomposition of azo-compounds (such as 2,2'-azobisisobutyronitrile, 2,2'-azobis(2-methylpropionamidine) dihydrochloride, dialkyl peroxides (such as tert-butyl peroxide and diacyl peroxides (such as benzoyl peroxide) in anaerobic nitric oxide (NO)-saturated dimethylsulfoxide (DMSO) or aqueous solutions yielded nitroxides. Well-characterized electron paramagnetic resonance spectra of nitroxides revealed that NO was favorable for reacting with carbon-centered and less stereoinhibited transient alkyl radicals, giving kinds of nitrosoalkane, typically nitrosomethane, which act sequentially as C-nitroso compounds to trap transient radicals present in solution, yielding spin-trapping adducts, i.e. nitroxides. Radicals, including sulfinyl radicals from UV-irradiated DMSO, were trapped by the in situ formed CH3NO. O-centered radicals could not add to the freshly formed C-nitroso compounds. Possible mechanisms are suggested.