Formaldehyde (СH2О) of 40% is widely used in medicine as a substance applied in processes for sterilization of bone grafts for further use as implants. The efficiency of sterilization depends on the amount of СH2О remaining in the graft pores [1]. Thus, the purpose of washing grafts with СH2О being the lack of СH2О at the end of the sterilization process. The purpose of the investigations is to establish the changes / response of the resistivity of ZnO-type semiconductor oxide films to the action of СH2О vapors, preventively subjected to rapid thermal treatment regimes. Similar investigations have been carried out on ZnO films doped with Ga and Sn, in which the mechanisms of the sensitization processes are discussed. [2,3]. The researches at different concentrations (5-1000 ppm) of СH2О vapor allowed to establish the concentration limit, which is obtained at the operating temperature of about 300 0C. Based on the results we propose the mechanism of sensitization to the circle of (СuО-Сu2О) -ZnО-Аg of the n type which have the sensorial features to vароrs of СH2О. Of the ZnO: Sn film samples, the ones most sensitive to СH2О vapors are those subjected to operation at a temperature of 400 oC (Fig.1). For all the measured samples, the sensitization of the СH2О vapors starts after 22-23 sec. Fig.1 Response to H2O and CH2O vapors of nanostructured films from ZnO-Ag (2), obtained for the purpose of excluding sensitivity to water vapors.