A biofilter study was conducted by changing inlet concentration and residence time for the removal of gaseous benzene and ethylene. In addition, carbon dioxide produced from the biofilters was investigated. Over 96% of benzene was removed at the residence times of 2 and 4.3 min, and inlet benzene concentrations of 220∼300 ppm. The ethylene biofilter was capable of achieving ethylene removal efficiency as much as 100％ at a residence time of 14 min, and inlet concentrations of 99∼290 ppm. At a steady state, the carbon dioxide of 409∼611 ppm was produced with an ethylene inlet concentration of 290 ppm. Most of benzene and ethylene were degraded at lower part of the biofilters where more microbial activity occurred.