To enhance the stability and selectivity of metal oxide conductivity sensors for organic gases in air a new sensor concept has been realized. A special chemical vapour deposition technique, using hydrolysis of tetraethylorthosilicate, was developed to produce selectively permeable SiO2 coatings of a few nanometres thickness on top of SnO2 detector films at 300-degrees-C. The SiO2 membranes were characterized with X-ray-excited photoelectron spectroscopy and secondary-neutral mass spectrometry. Electrical conductivity measurements at the uncovered as well as at the coated SnO2 were performed while an air flow with pulsed additions of various concentrations of methane and propane passed the sample. The SiO2 membrane enhances the sensitivity to methane by a factor of eight, whereas the sensitivity to propane is not affected. The response time was measured to be about 50 s with and without coating at concentrations larger than 500 ppm.