Carnosic acid, an antioxidant extracted from rosemary, is shown to produce radicals when in contact with oxidized methyl oleate in the absence of air above 50°C. Two radical species are formed: the first one, stable up to -110°C, is an hydroxy-phenoxy radical whose ESR spectrum was analyzed by studying its temperature dependence and its sensitivity to deuterium/proton exchange. The second species was observed above 110°C, its ESR spectrum was identical to the spectrum obtained when carnosol, another antioxidant extracted from rosemary, was heated at the same temperature in the presence of oxidized lipid. This observation is probably due to the transformation of carnosic acid into carnosol; the analysis of the corresponding ESR spectrum suggests the formation of a keto phenoxy radical exhibiting a great delocalization of the unpaired electron.