We report here for the first time bulk and grain boundary conductivities from impedance spectra of a ceramic proton conductor (BaZr0.7Pr0.2Y0.1O3-delta) taken during hydration and HID isotope exchange transients (at 400 degrees C). The results suggest that water moves quickly along grain boundary cores, and then interact from there with the space charge layers and, in turn, grain interiors. Hydration and HID isotope exchange have simple monotonic effects on the bulk conductivity in line with what is expected from it being dominated by protons. The transients for grain boundary conductivity exhibit however hysteresis: During hydration, the core charge and grain boundary resistance appear to go through transient minima related to non-equilibrium distributions of defects between the core and grain interior - notably because protons diffuse faster than oxygen vacancies between the grain boundary and grain interior. At equilibrium, hydration increases the core charge and the depletion of positive charge carriers in the space charge layers. During HID isotope exchange relatively fast hysteretic transients indicate that the space charge layers experience changes in charge carrier (D+ vs. H+) mobility as well as in D2O vs. H2O hydration thermodynamics. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.