Stable isotope ratios of individual plant components have become a valuable tool for the determination of the geographical origin and authenticity of foodstuff. A recently published method with considerable potential in this context is the measurement of the deuterium/hydrogen (D/H) isotope ratios of plant matter methoxyl groups. The method entailed cleavage of methyl ethers or esters with hydriodic acid (HI) to form gaseous methyl iodide (CH3I) and then measurement of the δ2H value of this gas. Here, as a follow up to a previous study, we describe a method for the rapid and precise δ13C analysis of plant matter methoxyl groups using gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS). Conditions for sample preparation were investigated for isotope discrimination effects, the GC conditions optimized, the reproducibility of the measurement of standards undertaken, and the precision of the method defined. The reproducibility of the δ13C value determined for a CH3I standard on 20 consecutive measurements was found to be 0.17‰. The method was also tested on four methoxyl-rich plant components: vanillin, lignin, wood and pectin. The analytical precision obtained, expressed as the average standard deviation, for these compounds was found to be better than 0.13‰. The described procedure which is simple and rapid, allowing preparation and analysis of a sample within 1 h, produces accurate and reproducible isotopic measurements. We suggest that this validated δ13C method when employed together with the recently published δ2H method for two-dimensional stable isotope studies of organic matter containing methoxyl groups will be of considerable value, e.g. for proving the authenticity of foodstuff. Copyright © 2009 John Wiley & Sons, Ltd.