Fine particles of perovskite-type SmFe 1−x Co x O 3 oxides were synthesized by the thermal decomposition of the corresponding heteronuclear hexacyano complexes. The sensing characteristics of ozone, methylethylketone (MEK), ethanol and benzene were examined. These sensors responded to sub-ppm levels of ozone, and the sensitivity decreased with increasing working temperature. The response to 0.1ppm ozone was ca. 1000 at 250°C. The conductance of the sensors in ozone decreased due to the contamination of the same VOC levels. At around 200°C, the cross sensitivity of some VOCs to ozone was quite depressed. The partial substitution of Co for Fe resulted in an increased conductance in air and a decrease in the sensitivity to ozone.