A high temperature van der Pauw method using a bulk disk sample was applied for a more reliable kinetic investigation in a protonic ceramic conductor, barium zirconate doped with 20 m/o yttrium. Similarly activated faster hydrogenation and slower oxygenation process can be explained by the moving boundary diffusion mechanism previously evidenced by in-situ optical spectroscopy where the hydrogenation kinetics is limited by the hydration front. The mechanism generally applies to all protonic ceramic conductors exhibiting non-monotonic conductivity relaxations. Defect-chemical consideration on the humidity, oxygen activity and temperature dependence of chemical diffusivities is provided.