Impedance spectroscopy has been used to characterize Sr0.97Ti1-xFexO3-delta materials. This technique was suitable to evaluate the grain and grain-boundary conductivities of samples with less than or equal to5% iron. The role of grain boundaries was changed by sintering in different conditions, and this was interpreted by accounting for microstructural differences. The corresponding relaxation times were also used to confirm the interpretation of complex impedance spectra. The values of grain-boundary conductivity and the corresponding values of activation energy obtained by this method were relatively close to predictions obtained on assuming a p --> n inversion at the grain-boundary core, as proposed by Waser and coauthors.(1-3) The temperature dependence of relaxation times confirmed this finding.