Abstract.(1-',C)-, (2-"C)-, (3-"C)-, (3-I3CH3)-, (4-I3C), and ("CH,O),-ubiquinone-10 and the corresponding (1-"C)-, (6-"C)-, (5-I")-, (5-I3CH,)-, (4-"C)-, and (13CH30)2-ubiquinone-0 have been synthesised from simple labelled starting materials via a single reaction scheme. The ubiquinones have been characterised using mass spectrometry, IH NMR and I3C NMR. The spectroscopic results indicate that, within experimental error, the syntheses have been accomplished without scrambling or dilution of label. All labelled ubiquinones have been synthesised on a decigram scale.