A method for the preparation of saturated fatty acids C-11 (C-13)-labelled in the omega-methyl position is described. A highly reactive zerovalent copper complex was prepared from lithium naphtalenide reduced lithium(2-thienyl)iodocuprate. The labelling precursors were obtained by addition of tert-butyl omega-iodocarboxylates to the organocuprate and these were reacted with [C-11]methyl iodide to form C-11-labelled.