An impedance spectroscopic investigation on a Gd-doped BaCeO3 single crystal is reported in which the electronic and ionic properties of the material have been studied systematically between 800 degrees C and 1200 degrees C under well defined oxygen partial pressures. For the first time, the conductance, the diffusion coefficients and the concentrations of the individual defects in a single crystal have been determined as a function of temperature and oxygen partial pressure. The ionic transport numbers are calculated in the light of the conductivity data. The defect structure of the material is discussed in the framework of point defect chemistry and the quasi-chemical reaction constants are determined. Based on the data acquired, the electron holes are suggested to be large polarons, whereas the electrons are small polarons.