Alkaline sodium dithionite reacts with solutions of C-nitroso-compounds to give stable radicals RN(SO2–)O·, the e.s.r. spectra of which are tabulated. With alkyl nitrites and with nitric oxide aqueous dithionite yields the nitrosyldisulphinate radical-anion, ·ON(SO2–)2 the e.s.r. spectrum of which is indistinguishable from that of the nitrosyldisulphonate radical-anion, ·ON(SO3–)2 of Frémy's salt.