We calculated the fraction of exchangeable hydrogen atoms in proteinaceous materials commonly analyzed for stable isotopic composition related to the region-of-origin of an animal. These included several types of α- and β-keratin, and muscle tissue. We find that the fraction of H atoms in keratin available for exchange at a biologically relevant temperature (25°C) averaged 9% across a range of ground organic materials, but was as high as ∼17% in cut hair; muscle tissue has ∼12% exchangeable H atoms. Under most analysis conditions, the difference in exchangeable fractions due to physical sample processing has a minimal effect on the calculated δ2H values of the non-exchangeable H atoms within a keratin-containing tissue (<2‰). However, extreme mismatches between sample and reference material types could affect δ2H values. Copyright © 2009 John Wiley & Sons, Ltd.