Thermogravimetric investigations on the perovskite Ba0.5Sr0.5Fe0.8Zn0.2O3−δ (BSFZ, with mixed hole, oxygen vacancy and proton conductivity) from water vapor can occur by acid–base reaction (hydration) or redox reaction (hydrogen uptake), depending on the oxygen partial pressure, i.e. on the material's defect concentrations. In parallel, the effective diffusion coefficient of the stoichiometry relaxation kinetics also changes. These striking observations can be rationalized in terms of a defect chemical model and transport equations for materials with three mobile carriers. Implications for the search of cathode materials with mixed electronic and protonic conductivity for application on proton conducting oxide electrolytes are discussed.