We investigated the d 13 C values of pyrolysis products from cellulose and lignin, the main components of wood, to prove that in the pyrolysis process no isotope exchange between different pyrolysis products appears and that hence pyrolysis products represent the isotope content of their precursors. Two cellulose samples and a lignin sample with d 13 C values of À23, À12 and À28%, respectively, were pyrolysed using Curie point pyrolysis simultaneously coupled to GC/MS and GC/IRMS. All samples were pyrolysed individually and in mixtures of lignin with both isotopically different celluloses in two mixing ratios, e.g. 1:1 and 4:1; the latter is representative for wood. No intermolecular isotope exchange between pyrolysis products from cellulose and lignin was observed. This proves that Curie point pyrolysis is suitable for investigating the isotope content of pyrolysis products in complex mixtures like wood. Pyrolysis products from both celluloses and lignin have d 13 C values similar to the bulk d 13 C value of the respective sample. In mixtures of cellulose and lignin d 13 C values of some pyrolysis products change slightly due to peak overlap and higher background signals. However, detected changes were independent from the isotopic difference between the mixing partners. Pyrolysis products such as 2,6-dimethoxyphenol, 4-methyl-2,6-dimethoxyphenol and trans-4-(2-propenyl)-2,6dimethoxyphenol for lignin and 2-hydroxymethyl-5-hydroxy-2,3-dihydro-(4H)-pyrane-4-one and levoglucosane for cellulose were found to represent the isotopic signature from cellulose and lignin in mixtures regardless of the composition of the mixture.