A novel method for the compound specific determination of Δ13C in VOC is presented. It combines a cryogenic preconcentration procedure with gas chromatographic separation, on-line combustion and isotope ratio mass spectrometry. The method allows the determination of Δ13C values of VOC at sub ppb levels with a precision of 0.5?. The procedure is suitable for routine operation. The accuracy of the method is of the same quality as the reproducibility of the measurements. The first ever measurements of Δ13C in selected VOC in clean air are presented and compared with data from samples of contaminated air. The variability in the Δ13C values of atmospheric VOC is much larger than the accuracy and precision of the measuring method. Hence the technique will be a very valuable tool to complement concentration measurements of VOC and to improve our understanding of the processes governing the distribution of VOC in the atmosphere.¿ 1997 American Geophysical Union