Ion conducting (= electron blocking) microelectrodes were used to measure the oxygen ion conductivity in mixed conducting oxides as a function of the thermodynamic activity of oxygen. The reported data concern mixed conducting perovskites of the composition La0.8E0.2CoO3 with E = Mg, Ca, Sr. The microelectrodes were made of yttrium-stabilized zirconia (radius 10-100 mum) Practical conditions and limitations of the microelectrode technique are described, e.g. the influence of the shape of the microelectrode, the microstructure of the interface and the pretreatment of the sample surface as well as further details for the measurement. Here, steady-state current-voltage curves were analyzed according to the Hebb-Wagner theory. The oxygen ion conductivity was calculated from these curves. The primary advantage of the microelectrode technique as compared to conventional planar contacts is the small effective diffusion length of about 10-100 mum due to the small electrode diameter and the radial geometry. Therefore, the time constants for approaching the steady state are two to four orders of magnitude lower as compared to a conventional thin sample with l-mm thickness (= diffusion length). (C) 2000 Elsevier Science B.V. All rights reserved.