Thin films of SnO2 were doped with 90 keV ions of Nb, Sb and Bi at a dose of 3 x 10(16) ions cm(-2). The implanted films were characterized by X-ray and ultraviolet photoemission spectroscopy, Auger depth profiling and sheet resistance measurements. Sb produces the most dramatic reduction in sheet resistance and is unique in introducing a sufficient concentration of electrons into the Sn 5s conduction band to be observable by photoemission. The response of the resistance of Sb-doped films to pulses of CH4, CO and H2O in a flow of synthetic air was investigated over a range of concentrations and temperatures. The films display relatively poor selectivity in their sensor response. By contrast, the more highly resistive Bi-doped films show marked selectivity towards CO relative to CH4 in sensor applications.