Tin oxide (SnO2) is a wide band semiconductor (Eg=3.6 eV) which is used for producing transparent electrodes and coatings for optoelectronic devices. Moreover one of the most successful applications of this material is as a sensitive element for gas sensors. However, in order to activate sensor properties of SnO2 additional heating up to T=100÷5000С is required what provided the thermal generation of free charge carriers (i.e. electrons). In this work an alternative mechanism of sensor properties activation by means of SnO2 illumination by UV light with phonon energy less than band gap of SnO2 is discussed. Films of SnOx (1≤х≤2) of 600 nm thickness investigated in this work were prepared by oxidation of tin covered on c-Si substrates by magnetron sputtering in Ar atmosphere. The obtained metallic films were annealed in air under temperatures ranges from 3500C to 7500C. According to transmission electron microscopy (TEM) and X-ray diffraction data the annealed samples have nanocrystalline structure and the average diameter of the nanocrystalls is about 100 nm. Raman spectra and IR-transmittance spectra were measured in order to investigate optical properties of the samples. It is known that electrical field of light wave may interact with free charge carriers (electrons, holes) what leads to appearing of characteristic reflection and transmittance spectra. In figure 1 differancial IRtransmittance spectra of the samples after 5 minute illumination by UV light of 380 nm wavelength and 1.4W power are shown. Moreover the spectrum of the sample heated to the temperature of 2500C in darkness is also presented. One can see that monotonic decrease of transmittance coefficient is observed after illumination (curves 2 and 4). The same changes of optical properties are also observed for the sample under heating in darkness (curve 3). Obtained results let us conclude that the modification of the IR spectra after UV illumination is related to an increase of free carrier concentration. It appears that mainly monopolar generation of free electrons takes place under samples illumination by the UV light. It seems that the effect observed in the samples which contain SnO2 phase can be explained by photoexcitation of the electrons from defect states. The phenomenon of free charge carrier increase wasn’t observed for SnO sample. The free charge carrier concentration were calculated from the IR data by using Drude model. It was shown that illumination by 380 nm UV light leads to fast saturation of the free charge carrier concentration up to N=1019 сm-3 during several minutes. Meanwhile the time of relaxation of free charge carrier concentration was about 40 minutes at room temperature. It was determined that the samples with the maximal content of SnO2 phase have the highest photosensitivity. The observed changes of free charge carrier concentration is the same with changes occurred under the samples heating up to the temperature of 2500C what points on the possibility of using photoexcitation in order to control operation mode of gas sensors based on tin oxide.figureFig 1 Differential IR transmittance spectra of SnOx films of different stoichiometry after additional treatment: after UV illumination at 220С for х≈1 sample (curve) and for 1.3<x<2 sample (curve 2); at Т=2500С for х≈2 sample in darkness (curve 3) and at Т=220С under UV illumination for х≈2 (curve 4).