The transport and catalytic properties of mixed conducting composites prepared from Gd0.7Ca0.3CoO3-delta (GCC) and Ce0.8Gd0.2O1.9 (CGO) were determined. In total, three compositions with 43, 60 and 75 vol.% CGO have been prepared. The composites have potential applications as oxygen permeable membrane in an amperometric sensor for NOx detection in exhaust gases. At all compositions, three different phases were found in the annealed composite as a result of solid state reactions during annealing. The gadolinium content of the CGO phase was increased. The GCC was transformed into a phase with K2NiF4-structure and an additional CoO phase appeared. Electron conductivities were determined in the range 100 to 750 degreesC. The oxygen ion conductivities were measured by a microelectrode technique between 650 and 750 degreesC. Permeation measurements were carried out between 850 and 1000 degreesC. Effective ionic conductivities were calculated from the permeation results. The latter agreed well with the microelectrode data extrapolated to higher temperatures. The composition dependence of the conductivities was explainable by a statistical percolation model. The investigated composites exhibit good oxygen ion conductivities. Typical ionic conductivity values for a composite with 75 vol.% CGO increased from 4 x 10(-4) (Omega cm)(-1) at 650 degreesC to 4.2 x 10(-2) (Omega cm)(-1) at 1000 degreesC. The heterogeneous catalysis of surface gas reactions was tested by exposing powdered samples to various gaseous mixtures with O-2, N-2, CO, CO2, H2O, SO2 and propene at temperatures up to 800 degreesC. No conversion of NO to oxygen and nitrogen as well as no extensive formation of N2O was detected. It is concluded that the composite membranes are suitable for the intended applications.