The total conductivity and the partial hydrogen ion conductivity of a single crystal of SrTiO3 have been investigated by transport number measurements using a hydrogen activity concentration cell/EMF method in wet atmospheres as a function of pO(2) (10(-20)-1 atm) at 1000 degrees C and as a function of temperature (350-1050 degrees C) in wet hydrogen. The single crystal exhibits a small but readily measurable proton conductivity contribution in wet oxidising atmospheres. Earlier indications of apparent - but unlikely - transport by negatively charged hydrogen ions under reducing conditions and high temperatures in polycrystalline SrTiO3 have been reproduced for the single crystal, thus eliminating grain boundaries and porosity as a cause of such findings. The paper proposes transport of neutral hydrogen species through the oxide as an alternative cause: This results in polarization of the hydrogen activity at the interface between the electrodes and the electrolyte, in turn giving rise to an unintended gradient in oxygen activity and an EMF resulting from the relatively high oxygen ion transport number of the material. (c) 2006 Elsevier B.V. All rights reserved.