Perovskite oxides containing multivalent cations and oxygen non-stoichiometry exhibit mixed ionic/electronic conductivity, enabling oxygen exchange with the gas phase at elevated temperatures. This “breathing” behavior underlies performance of fuel/electrolysis cell electrodes, gas separation membranes, and thermochemical reactors. Here, we explore the coupling of oxygen exchange with two processes: 1) excess carrier generation by illumination and 2) crystallization. In the former Sr(Ti,Fe)O3-x thin films are utilized to explore photo-ionic effects in thin films with surface exchange kinetic limitations; our results demonstrate oxygen incorporation from the gas phase under UV illumination. For the latter, our work on crystallization of amorphous-grown (La,Sr)(Ga,Fe)O3-x thin films explores the consequences of oxygen incorporation from the gas phase during crystallization for electronic and ionic conductivity. The material transitions from a predominantly ionic conductor in the amorphous phase to a predominantly electronic conductor as crystallinity increases. These effects can be understood in terms of non-equilibrium evolving defect chemistry.