The aim of the present work is to test the performance of thin films of NiO obtained by vacuum technology to toxic gases. The first gas considered is NO2 due to the fact that it is toxic to human body and harmful to the environment. NiO thin films were deposited by RF reactive magnetron sputtering on alumina substrates provided with Pt interdigitated microelectrodes and a Pt heater on the reverse side. The samples were annealed at 600 degrees C, 700 degrees C and 800 degrees C in order to compare their response to the selected gas. XRD and energy dispersive spectroscopy (EDS) analysis were carried out to correlate the different electrical responses to the material microstructure. Moreover, morphological characterization of the sensing films was performed by field emission gun-scanning electron microscopy. Two different thicknesses were deposited (150 nm and 300 nm) in order to study the influence of this parameter. The samples were exposed to concentrations between 5 and 100 ppm of NO2. The 150 nm-thick samples at 700 degrees C showed the best responses to low concentrations at 300 degrees C.