A d.c. gas discharge activating reaction evaporation technique was used to prepare gas sensitive ZnO and SnO2 bilayer films. The bilayers were grown successfully onto glass substrates and the optimum processing conditions were obtained. Here, the main crystal phases of ZnO/SnO2 and SnO2/ZnO are SnO2 and SnO were with preferential growth in the (002) and (111) directions, while ZnO phase exhibits an amorphous phase. The bilayers were characterized by X-ray diffraction, scanning electron microscope showing ultrafine particle (UFP), and their gas sensing properties were also investigated. The UFP bilayers with average surface particle size of 45-80 nm exhibited a good sensitivity of 7.8 at 330 degrees C (SnO2/ZnO), 9.0 at 290 degrees C (ZnO/SnO2) to 2000 ppm C2H5OH, a very good selectivity for detecting C2H5OH in the presence of gasoline gas and acetone gas, a decrease of operating temperature, and also a fast response below 15 and 20 s to C2H5OH at their optimum operating temperature, respectively. The effects of annealing on topography and crystal phase of the bilayers were discussed. Meanwhile, their interface was observed by a scanning electron microscope method, and the results show that the bilayers have an obvious diffusion and clear columnar structure.