Key operating principle of semiconductor gas sensors is based on high sensitivity of surface electrical properties to the ambient atmosphere composition. An adsorbed molecule or atom can trap a free electron from the sensor bulk, thus decreasing its conductivity. For practical applications, it is important to provide high rates of both adsorption and desorption of atoms and molecules to and from the surface. These processes can be stimulated by excessive generation of electron-hole pairs either by thermal heating or UV irradiation, both of which increase substantially the power consumption. A reduction of power consumption is possible by using the irradiation in the visible spectral range. Bulk metal oxide semiconductors (MOx) which are commonly used as gas sensors are transparent in this spectral region. In this work, we study possibilities for sensitization of MOx structures with colloidal CdSe nanocrystals.   Nanocrystalline metal oxide semiconductors ZnO, SnO2, In2O3 were synthesized by the precipitation method. Characterization of MOx was carried out by the X-ray diffraction (XRD) analysis and the scanning electron microscopy. Colloidal CdSe quantum dots (QD) with 2-5 nm size were synthesized by a high-temperature method using the oleic acid as a stabilizer. The structure of QD was studied using the XRD and transmission electron microscopy. Sensitization was achieved by direct adsorption of CdSe nanocrystals on the MOx surface. Mechanical linking of QDs onto the surface of MOx was confirmed by inductively coupled plasma mass spectrometry, high-angle annular dark-field imaging and energy dispersive X-ray spectroscopy.   Temperature dependencies of conductivity for MOx in the 300-77 K temperature range indicate the barrier-limited conductance mechanism with barriers ~40 meV. The conductivity is exponentially dependent on the surface charge, making the structures extremely sensitive to variation of the carrier concentration. This feature makes the structures under study good candidates for gas sensors. Photoconductivity spectra taken in the 750-400 nm range for composite MOx-QD structures confirm charge carrier transport between QDs and MOx. The photoconductivity is persistent, and its analysis shows that that QDs in composite structures act as both injection and recombination centers with complex energy spectrum. Embedding of QDs into MOx results in a shift of local absorption maxima corresponding to QD’s energy spectrum. The change in amplitude and sign of the shift with decreasing the QD size is studied and explained by partial oxidation of the QD surface and by the ligand interaction.   Sensor measurements were performed by means of exposure to the 0.2 – 1.6 ppm NO2 mixture in the dry air under periodic green light illumination for nanocrystalline ZnO, SnO2 and In2O3 films and composites with embedded QDs. O2 sensing was studied, too, since NO2 competes with oxygen for the same adsorption sites. Dependence of the electric resistance on the gas concentration shows that MOx-QD structures under visible light illumination have comparable sensing amplitude and speed with bulk MOx sensors under UV illumination. Thus MOx-QD structures can be used for NO2 detection under visible light illumination at the room temperature without any heating, while retaining the traditional heating or UV excitation possibilities as an additional control channel. The mechanism of operating principles for MOx-QD structures is suggested and discussed.