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NiO thin films deposited by RF reactive magnetron sputtering at room temperature were characterized by low optical transmittance, high acceptors concentration and low mobility of carriers. Storage of NiO films in air atmosphere and at elevated temperature leads to degradation of semiconducting conductivity of NiO. To improve NiO film stability the samples were preserved from air by covering them with thin SiO2 film. Influence of SiO2 capping layer on mentioned properties after annealing in argon and oxygen ambient at temperatures from 200°C up to 600°C were examined. High temperature was applied to study thermal bleaching of NiO films with SiO2 capping layer. Optical transmittance as well as bandgap are increased after subsequent annealing at 350°C and rise much more significantly after annealing at higher temperature. The electrical parameters of NiO film without cap are drastically changed after annealing in Ar even leading to huge resistivity, but these SiO2 covered films reveal increased resistivity and improved mobility above 1 cm2/V·s after annealing at 350°C.