An unusual dual conductance response was found in the case of exposure of n-type semiconductors CdS and α-SnWO4 in the form of thin films to carbon monoxide. Usually, CO behaves as a reducing gas increasing the conductance of n-type semiconductors. However, at temperatures below a border temperature T‒, CO did behave as an oxidizing gas decreasing the conductance of CdS and αSnWO4 thin films. T‒ for this dual conductance response to CO was found to be below 50 °C for CdS films. The enrichment of the surface with cadmium species had a decreasing effect on T‒ which was also decreasing with decreasing partial pressure of CO. For α-SnWO4 films, T‒ was near 250 °C depending on the partial pressure of CO. It was found that the dual response behaviour depends on temperature, CO concentration in air and tin content in the film. © 1994 IOP publishing Ltd.