Hydrogen first, oxygen next : The kinetics of water incorporation in situ has been followed by spatially resolved local optical absorption spectroscopy as well as by integral conductance by using SrTiO 3 as a model oxide proton conductor. The surprising nonmonotonic redox kinetics can be explained by a decoupling of the H 2 O diffusion into a fast diffusion of hydrogen and a more sluggish diffusion of oxygen (see picture).