Pure nitric oxide does not add to alkenes containing acceptor or donor substituents, or to conjugated dienes, to afford β-nitrosoalkyl radicals. EPR spectra show that reactions are initiated by NO₂ addition to carbon–carbon double bonds to produce β-nitroalkyl radicals which combine with nitric oxide to yield β-nitro-nitroso-compounds. The latter trap other radicals to afford mixtures of aminoxyl radicals and, where possible, the nitroso-compounds tautomerise to oximes which oxidise to iminoxyl radicals. EPR spectra obtained on reaction of NO–NO₂ mixtures with 2,5-dimethylhexa-2,4-diene have shown the presence of a di-tert-alkylaminoxyl plus a tert,sec-dialkylaminoxyl, providing good support for this mechanism. Product analyses have established that 2-oxo-4-methylpent-3-ene and 1-acetylcyclohexene undergo extensive oxidative degradation. For enol ethers, reaction with NO–NO₂ leads to hydrolysis giving the corresponding carbonyl compound and alcohol, followed by ketal and ester formation.