We report the microstructure and gas-sensing properties of a nonequilibrium TiO₂-SnO₂ solid solution prepared by the sol-gel method. In particular, we focus on the effect of Cd doping on the sensing behavior of the TiO₂-SnO₂ sensor. Of all volatile organic compound gases examined, the sensor with Cd doping exhibits exclusive selectivity as well as high sensitivity to formaldehyde, a main harmful indoor gas. The key gas-sensing quantities, maximum sensitivity, optimal working temperature, and response and recovery time, are found to meet the basic industrial needs. This makes the Cd-doped TiO₂-SnO₂ composite a promising sensor material for detecting the formaldehyde gas.