Application of a material with the nominal composition Gd0.7Ca0.3CoOx is considered for use as membrane material in an exhaust gas sensor for NOx. SEM-EDX and XRD measurements revealed that after sintering at 1200 degreesC in air, three co-existing phases are present: (Gd0.6Ca0.4)(2)CoOx (60 vol,%), GdCoO3 (26 vol.%) and CoO, Catalytic activities were investigated using temperature-programmed reaction (TPR). Whereas the composite Gd0.7Ca0.3CoOx shows no catalytic activity towards NO, reduction, as required in the targeted application, the single phase components (Gd0.6Ca0.4)(2)CoOx and GdCoO3, were found both to be active. The permeation rates exhibited by the composite material, at 1000 degreesC, are considered too low for real application of this material. Oxygen permeation rates through perovskite-structured GdCoO3-delta and (Gd0.6Ca0.4)(2)CoO4-delta, having the K2NiF4-structure, were found to be below the range of detection (< 10(-11) mol/cm(2) s). <(c)> 2001 Elsevier Science B.V. All rights reserved.