The paper describes some reaction engineering fundamentals of the separation of organic air pollutants (volatile organic compounds) from waste gases using fixed-bacteria monocultures (biocatalysts) in a trickle-bed reactor. In particular the influence of pollutant concentration and oxygen concentration are investigated. The separation efficiency of certain substances such as acetone and isopropanol depends strongly on the oxygen concentration. The results obtained can be described by a mathematical model based on the diffusion of oxygen into the biofilm (diffusion regime of the catalyst). The non-stationary operation of the reactor-interruption of the oxygen stream and strong fluctuation in the exhaust gas stream-showed that other components such as propionaldehyde and n-propanol could be eliminated for a certain time without oxygen. Propionic acid is formed.