A packed granular activated carbon (GAC) biobed, inoculated with the ethene-degrading strain Mycobacterium E3, was used to study ethene removal from a synthetic waste gas. Ethene, for which the dimensionless partition coefficient for an air-water system at 20 degrees C is about 7.6, was used as a model compound for poorly water soluble gaseous pollutants. In a first mode of operation, the GAC biobed was sprinkled intermittently and the waste gas influent was continuously pre-humidified, establishing relatively moist conditions (water content >40% to 45%). A volumetric ethene removal rate of 0.382 kg COD.m(-3).d(-1) (0.112 kg ethene.m(-3).d(-1)) was obtained for an influent concentration of 125 ppm, a superficial waste gas velocity of 3.6E-3 m.s(-1) and a pseudo residence time of 45 s. However, in the second mode of operation, omitting the pre-humidification of the waste gas influent and establishing a "dry" biobed (water content <40% to 45%), and thus obtaining better mass transfer to the biofilm, the ethene removal could be doubled for otherwise comparable operating parameters. Furthermore, under decreased wetting and for the given experimental conditions (influent concentration 125 to 816 ppm, waste gas superficial velocity 3.0E-3 m.s(-1), pseudo waste gas residence time 43 s), the ethene removal was not limited by mass transfer of ethene through the water layer covering the biofilm.