We have obtained oxygen-17 (17O) nuclear magnetic resonance (NMR) spectra of C17O ligands bound to ferrous horseradish peroxidase isozyme A, isozyme C, and ferrous chloroperoxidase, as a function of pH. Our results show that the peroxidases exist in two distinct states, the acidic and alkaline forms, which undergo reversible acid-base-induced transitions characterized by a single pK value. The two forms are characterized spectroscopically in much the same way in all three proteins, suggesting a similar structural origin for the transition process. In particular, the 17O NMR signal of the acidic form is approximately 7 ppm more shielded than that of the alkaline form, and the CO ligand in the acidic form appears to have a smaller 17O nuclear quadrupole coupling constant than that of the alkaline form. We have also obtained the pK values and exchange rates for all three peroxidases. The results indicate that a similar structural change may be involved in the transition process in all three peroxidases.