Substance mixtures comprising (A) vinyl-containing compounds, (B) an active amount of a mixture inhibiting the premature polymerization of the vinyl-containing compounds, comprising (i) at least one N-oxyl compound of a secondary amine which carries no hydrogen atoms on the .alpha.-C atoms, and (ii) at least one iron compound, (C) if appropriate nitro compounds, and (D) if appropriate costabilizers, a process for inhibiting the premature polymerization of vinyl-containing compounds (A) and the use of mixture (B), if appropriate as a mixture with nitro compounds (C) and/or costabilizers (D), for inhibiting the premature polymerization of free radical polymerizable compounds and for stabilizing organic materials against the damaging action of free radicals.