The excess noise in commercial semiconductor gas sensors, during exposure to different gases at various temperatures, was studied. The measurements have been carried out using different concentrations of ethanol vapor, CO, NOx, H2 and SO2 gases in synthetic air. The normalized noise spectra of the sensors stabilize after the first day of the burning-in process in synthetic air. The sensors do not show significant deviation from the ohmic behavior over the full range of applied voltage. The induced excess noise is proportional to the square of the applied voltage, so resistance fluctuations explain the spectra. Each gas induces a characteristic noise spectrum. The analysis of resistance fluctuations in gas sensors, during exposure to gases, has the potential of a strongly enhanced sensitivity and selectivity.