A solid-state galvanostatic polarization method is presented for the simultaneous determination of the chemical diffusion coefficient (D) over tilde and the ionic conductivity sigma(i) of mixed conducting oxides as a function of nonstoichiometry. The method is especially designed for oxides which react with the solid electrolyte. With the symmetrical electrochemical cell used, the sample is located between two ionic electrodes separated by thin porous platinum interlayers to avoid any side reactions. Polarization measurements are applied to homogeneous samples of La0.4Sr0.6CoO3-delta,5 in the pO(2) range between 10(-4) and 10(-1) bar (3-delta=2.68-2.81) at 825degreesC. (D) over tilde decreases with increasing oxygen content. sigma(i) shows a maximum at 3-delta=2.77 and decreases with decreasing oxygen content indicating the possible formation of vacancy ordered structures. Values for (D) over tilde from the polarization measurements at T=825degreesC and from oxygen exchange measurements at the same temperature are in good agreement with each other.