Abstract: A method of gas detection and differentiation between various noxious gases including exposing a semi-conductor catalytic sensor in the atmosphere to be evaluated, sequentially providing first and second voltage level inputs to the sensor, and determining the relative change in the output of the sensor as said input voltage is changed from the first level to a second level. A reduction in the output of the sensor in response to the increase in input voltage corresponds to the presence of carbon monoxide and the absence of hydrocarbons in the atmosphere being evaluated. An increase in the sensor output in response to an increase in the input voltage indicates the presence of hydrocarbons in the atmosphere being evaluated.