Nanocrystalline powders of Zn1-xCoxAl2O4 (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) mixed oxides, with cubic spinel structure were successfully prepared by the ethylene glycol mediated citrate sol-gel method. The structure and crystal phase of the powders were characterized by X-ray diffraction (XRD) and microstructure by transmission electron microscopy (TEM). X-ray diffraction results showed that the samples were in single phase with the space group Fd-3m. TEM analysis showed that the powders with spherical shape were uniform in particle size of about 17-24 nm with mesoporous in nature. Further investigations were carried out by FT-IR. Thick films of as-prepared Zn1-xCoxAl2O4 powders were fabricated using screen-printing technique. The response of Zn1-xCoxAl2O4 based thick films towards different reducing gases (liquefied petroleum gas, hydrogen, hydrogen sulfide, ethanol gas and ammonia) was investigated. The sensor response largely depends on the composition, temperature and the test gas species. The Co (cobalt) content has a considerable influence on the gas-sensing properties of Zn1-xCoxAl2O4. Especially, Zn0.4Co0.6Al2O4 composition exhibited high response with better selectivity to 100 ppm C2H5OH gas at 150 degrees C. The instant response (similar to 7 s) and fast recovery (similar to 16 s) are the main features of this sensor. (C) 2011 Elsevier B. V. All rights reserved.