A gas sensor of SnO2-based materials has been made by thick-film technology utilizing hydrophobic silica as a binder. The technology can achieve a high productivity as well as a reduced humidity dependence and a sufficient mechanical strength of sensors. A thick-film sensor of SnO2 incorporated with ThO2 shows highly selective detection for CO gas, separated from H2 gas; i.e., sensitivity to CO is 42 times higher than to H2 at both gas concentrations of 50 ppm. An excellent humidity-independent sensitivity is also achieved. Copyright © 1979 by The Institute of Electrical and Electronics Engineers, Inc.