The electrical conductivity of sputtered 2 m thick films of the oxide semiconductor B-Ga203 was studied in function of the partial pressure of CO or Hz above the solid and also in the presence of CO +O,, H,+Q and CO +H, mixtures. The concentrations of the examined gases varied in the range 0.01-l vol.% in nitrogen carrier gas, while the experiments were carried out at 823, 873 and 923 K. Under the influence of Hz and CO the conductivity of the samples increased with the partial pressures of the gases according to the relations: Ig cr* (l/n)lgp, and lg o-(l/m)lgh, where II and m are greater than unity. As a second step the effect of the coadsorption of CO tOz and H2+Oz on the conductivity was investigated. The &GaZ03 sensors, connected electrically by Pt electrodes, manifested a moderate catalytic activity. Due to this property the conductivity of the solid changed steeply in the vicinity of the stoichiometric composition of the gas mixtures. In the third step the coadsorption of CO t H, was studied. In function of the CO partial pressure the conductivity of the solid passed through a minimum. The shape of the experimental curves was attributed to the formation of certain organic intermediate surface compounds.