Columnar bioreactors are being tested for the treatment of volatile organics. The bioreactors contain microorganisms that are able to remove and degrade the dilute gaseous hydrocarbons (n-pentane and isobutane) from effluent air streams by biological action. Columnar bioreactors, including a liquid continuous packed column, were continuously operated for over 18 mo with sustained degradation of the gaseous hydrocarbons. In these systems, the hydrocarbons, pentane, and isobutane are the sole carbon and energy sources for the microbes. The overall conversion rate was increased by operating at higher gas flow rates. This indicates the importance of improving mass transfer and gas contact in these systems.