A NMR approach to in situ precise determination of the proton exchange rate in water molecules sorbed on microporous crystals is presented and applied to hydrated zeolite chabazite and its various ion-substituted forms. This approach has allowed us to measure a frequency of proton exchange in the range from 2 x 10(1) to 2 x 10(3) s(-1). The observed exchange is interpreted as a result of the proton-transfer reaction between H2O molecules and chemical defects represented by Bronsted acidic sites. For the substances studied, the barrier for this reaction is found to vary in the range 45-60 kT/mol.