A method for inhibiting premature polymerization of vinyl aromatic monomers

The invention pertains to a process for inhibiting the premature polymerization of a vinyl aromatic compound during distillation or purification which comprises incorporating therein a stable cyclic hindered nitroxyl compound used together with oxygen or air to enhance the inhibiting activity of the nitroxyl compound, with the proviso that no nitro-aromatic compound is present and the cyclic hindered nitroxyl compound is not (2-butyl-3-(1-oxyl-2,2,6,6-tetramethyl-piperidine-4-yloxycarbonylamino)-phenyl)-carbamic acid 1 -oxyl-2-2,2,6,6-tetramethyl-piperidine-4-yl ester or (6-(1-oxyl-2,2,6,6-tetramethyl-piperidine-4-yloxycarbonylamino)hexyl)-carbamic acid 1-oxyl-2,2,6,6-tetramethyl-piperidine-4-yl-ester.