The instant invention relates to stable free nitroxyl radicals of formula (I) wherein at least one of the substituents R is —O. and the others are hydrogen or OH; X is —NR1R2, wherein R1 and R2 are independently hydrogen, C1–C18alkyl or together with the nitrogen atom to which they are bound form a 5 or 6 membered ring which may be further interrupted by an O atom. Further subjects of the invention are an oxidation process for alcohols to aldehydes or ketones or to carboxylic acids in the presence of a compound of formula (I) and the use of stable free nitroxyl radicals of formula (I) as oxidation catalysts.