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In this work nickel oxide (NiO) thin films have been tested against low concentrations of formaldehyde (HCHO). NiO thin films were deposited on alumina substrates by RF reactive magnetron sputtering in a mixed atmosphere of argon and oxygen. A Pt heating resistor was deposited on the reverse side for an accurate control of the operating temperature. Samples were annealed in synthetic air for 4 hours at 700°C in order to stabilize their microstructure. Two different thicknesses (150 and 300 nm) were deposited in order to study the influence of this parameter on both the microstructure and sensor response. Both XRD analysis and FEG-SEM images show a smaller grain size for the 150 nm-thick samples. The best operating temperature was established at 340 and 300°C for 150 and 300 nm-thick samples respectively. A higher sensitivity was obtained for the samples of lower thickness for a set of HCHO concentrations ranging from 5 to 20 ppm. Moreover, the repeatability of the experiments was tested for the most sensitive samples.