Nanostructured NiO thin films were prepared by dc reactive magnetron sputtering in a mixture of oxygen and argon and subsequently coated by Pt on an NiO film surface. Pt very thin overlayers with a thickness of about 3 and 5 nm have been deposited by magnetron sputtering. The microstructure and surface morphology of the samples have been analysed by XRD and by SEM and AFM, respectively. The electrical responses of the NiO-based sensors towards hydrogen concentration have been also considered. The thickness of the Pt thin layers seems an important parameter in determining the properties of the NiO films as hydrogen sensors.