An experimental study of the processes occurring during protonation of bulk ceramic proton conductors, BaCe0.9Y0.1O2.95 (BCY10) and BaCe0.8Y0.2O2.9 (BCY20), is presented here. One of the faces of a disc-shaped diffusion membrane was connected to a mass spectrometer in a vacuum system permitting the identification of the species crossing the ceramic-vacuum interface. Exposing the sample to D2O led to a strong signal of D2O+ after a certain lag time. From these lag times, the tracer diffusivity of hydrogen could be determined as a function of temperature. The permeation of steam consisted of two components: a fast component, essentially given by the diffusivities of deuterons (protons), and a slow component, ascribed to chemical diffusion of deuterons (protons) coupled to oxygen vacancies with the possibility of participation of even more complex defects. Dilatometry measurements of different specimens of BCY10 and BCY20 also revealed quite clearly this two-phase pattern during protonation. Diffusion measurements on protonic ceramic membranes using (H2O)-O-18 permitted the determination of the tracer diffusivity of oxygen. All of the above measurements were interpreted in the light of the chemical diffusion model developed by Kreuer et al. (C) 2004 Elsevier B.V. All rights reserved.