We present new gas-sensitive material that has been obtained by mixing the powders of tin dioxide (semiconductor) and ruthenium dioxide (metallic conduction). The powder mixture was doped with platinum by the impregnation method. The X-ray and scanning electron microscopy investigation has shown that this material is a three-phase composition. The resistance of (SnO2 + RuO2 + Pt) material is much higher than pure SnO2. Applied as a gas-sensitive layer in thick-film sensors, the material presents high selectivity to methane in the presence of carbon monoxide. The three-phase composition reacts in a way similar to SnO2 when applied as a catalyst in a test reaction of n-butyl alcohol condensation. Considering the microstructure, numerous research studies, and literature, the authors suggest the following explanation of physicochemical effects appearing in this gas-sensitive material. The authors think that two potential barriers appear in this composition: the first barrier on the boundary of tin dioxide grains, the second one is Schottky's barrier on the boundary of SnO2 grain and RuO2 grain. The appearance of Schottky's barrier on SnO2/RuO2 boundary causes an increase in resistance of studied composition. The definite improvement in sensor selectivity is caused by specific catalytic properties of RuO2. The catalyst reveals high activity in the heterogeneous reaction of carbon monoxide conversion by water vapor. Analyzing the results of conducted research, the authors think the selectivity of resistive gas sensors may be significantly improved by applying multiphase gas-sensitive material. (c) 2006 The Electrochemical Society.