Mixed ionic and electronic conductors (MIECs) are a highly relevant material class in the field of solid-oxide cells and are, for example, promising candidates for electrodes with fast interfacial reaction kinetics. While there are many studies dealing with the bulk conductivities of such MIECs, models describing the interfaces between two mixed-conducting oxides have been far less developed. This study focuses on the investigation of space charges at the interfaces of the model perovskite SrTiO3 with different MIECs. Impedance spectroscopic measurements at 500 °C revealed that the MIECs under investigation can be divided into materials leading to negligible (YBa2Cu3O7−δ), moderate [(La,Sr)FeO3−δ, (La,Sr)CoO3−δ], and large [(La,Sr)MnO3−δ, (La,Sr)CrO3−δ] space charge resistances in SrTiO3 single crystals. The fundamental cause for these different space charge resistances is different space charge potentials, and we show that these can be determined by various methods with excellent agreement, ranging from X-ray photoelectron spectroscopy to impedance spectroscopy and photovoltage measurements. A model is introduced to correlate the ionic and electronic driving forces determining the space charges and to predict the space charge potentials from the electronic and ionic bulk properties of the corresponding mixed-conducting oxides. This model is also used to relate space charge potentials with reducibilities of MIECs, i.e., transition points from hole to vacancy compensation of an acceptor dopant in defect chemical Brouwer diagrams. The predicted trends are in good agreement with thermodynamic data on defect formation energies from the literature. Accordingly, the given model provides a widely applicable framework to predict and describe the space charge properties of a variety of MIEC heterojunctions.