To determine the 13C abundance of combustion and break down products formed in cigarette smoke, especially CO and CO2, a simple and fast analytical method is needed. Taking into account the knowledge about the determination of the natural 13C abundance in air, an online method - based on gas chromatography-reaction-continuous flow mass spectrometry (GC-R-CF-MS) - has been developed, which enables the determination of the 13C abundance of CO and CO2 in the vapour phase of cigarette smoke with a relative standard deviation of< or =0.5% in one analytical run. Additionally, in a second step, the 13C abundance of total volatile carbon can be determined.