Laws and regulations for the reduction of solvent emissions tend to force increasingly lower limits on gaseous emissions. Biological waste-gas treatment is an ecological and rather cheap method for solvent reduction, which is often used for waste-gas streams containing two or more components. The microbiological degradation of toluene and heptane as well as of several mixtures of these substances were investigated in a laboratory plant trickling-bed reactor. It was found that toluene was degraded at up to three times the rate for the heptane. Heptane elimination in the mixtures was low and the toluene elimination rate at low organic loads approached 100%.