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Breath and diet samples were collected from 29 taxa of animals at the Zurich and Basel Zoos to characterize the carbon isotope enrichment between breath and diet. Diet samples were measured for δ 13 C and breath samples for CH 4 /CO 2 ratios and for the respired component of δ 13 C using the Keeling plot approach. Different digestive physiologies included coprophagous and non-coprophagous hindgut fermenters, and non-ruminant and ruminant foregut fermenters. Isotope enrichments from diet to breath were 0.8 ± 0.9‰, 3.5 ± 0.8‰, 2.3 ± 0.4‰, and 4.1 ± 1.0‰, respectively. CH 4 /CO 2 ratios were strongly correlated with isotope enrichments for both hindgut and foregut digestive strategies, although CH 4 production was not the sole reason for isotope enrichment. Average CH 4 /CO 2 ratios per taxon ranged over several orders of magnitude from 10 –5 to 10 –1 . The isotope enrichment values for diet-breath can be used to further estimate the isotope enrichment from diet-enamel because Passey et al. (2005b) found a nearly constant isotope enrichment for breath-enamel for diverse mammalian taxa. The understanding of isotope enrichment factors from diet to breath and diet to enamel will have important applications in the field of animal physiology, and possibly also for wildlife ecology and paleontology.