Increasingly, the entrance air of car passenger space is purified from petrol hydrocarbons through adsorption onto active carbons in thin-layer filters. In this publication some active carbons are characterized by fixed-bed adsorption measurements of 80 ppm n-butane with three different air humidity levels; the variation of butane capacity is discussed in terms of water vapour and nitrogen adsorption isotherms on the same carbons. With 70% relative humidity, an excess concentration of butane concentration over the inlet is observed because of displacement by water.