The δ 1 3 C- and δ 1 5 N-values of caffeine and theobromine from various natural origins have been determined. In agreement with earlier findings, they permit distinction from corresponding synthetic products. For a 1 3 C pattern analysis, the purine alkaloids were hydrolysed by means of 22.5 M H 2 SO 4 , and the relative 1 3 C abundance of the fragments was analysed by GC-C-IRMS. The 1 3 C pattern of natural purine alkaloids indicated an enrichment of 1 3 C in the positions 2 and 8, originating from the THF pool (δ 1 3 C = - 21, 7‰) and a depletion in the methyl groups coming from the SAM pool (δ 1 3 C = - 39‰). The results suggest the existence of two separate C 1 pools and an isotope effect implied in the formation or transfer of CH 3 groups. The carbon isotope pattern of synthetic purine alkaloids was completely different from that of the natural alkaloids.