ZnO films grown by a home-made Aerosol Spray Pyrolysis (ASP) system at 350 degrees C have shown a characteristic columnar structure with an inhomogeneous; layer structure that extends all the way to the surface. Surface morphology studies revealed grain sizes of the order of 80 nm and a highly porous structure which proved catalytic for the performance as gas sensing element. Conductometric ozone response tests have shown that the films of such a structure may be very effective in detecting ultra low ozone concentrations. The lowest ozone limit detected was 16 ppb at room temperature opening up the road for ultra sensitive inexpensive metal oxide ozone detectors. (C) 2009 Elsevier B.V. All rights reserved.