Fuel vapor retaining systems in cars normally are using activated carbon filters to prevent vapors from being emitted to the environment. For regeneration of these filters air is soaked from the ambient and after being loaded with organic vapors sent to the engine. As this air is wet, water can adsorb and cumulated on the filter over the years and may reduce the sorption capacity of the filter for organic vapors. Coadsorption measurements of n-pentane vapor-humid air mixtures on pre-wetted activated carbon have been performed. The mass of the coadsorbate pentane-water mixture nearly equals the sum of the pure component adsorbates. The loading capacity of the activated carbon for n-pentane is not significantly reduced by the pre-adsorption of water.