Kinetic and equilibrium isotope effects in peptide group hydrogen exchange reactions were evaluated. Unlike many other reactions, Kinetic isotope effects in amide hydrogen exchange are small because exchange pathways are not limited by bondbreaking steps. Rate constants for the acid-cat-alyzed exchange of peptide group NH, ND, and NT in H2O are essentially identical, but a solvent esotope effect doubles the rate in D2O. Rate constants for base-catalyzed exchange in H2O decrease slowly in the order NH > ND > NT. The alkaline rate constant in D2O is very close to that in H2O when account is taken of the glass electrode pH artifact and the difference in solvent ionization constant. Small equilibrium isotope effects lead to an excess equilibrium accumulation of the heavier isotopes by the peptide group. Results obtained are expressed in terms of rate constants for the random coil polypeptide, poly-DL-alanine, to provide reference rates for protein hydrogen exchange studies as described in Bai et al. [preceding paper in this issue]. © 1993 Wiley-Liss, Inc.