A fixed film spiral bioreactor containing immobilized activated sludge microorganisms has been used to degrade ethanof vapors. The effect of air flow rate, and ethanol feed concentration on elimination capacity has been investigated. Air flow rate is varied in the range from 2.34 to 40.0 dm(3) min(-1). Ethanol feed concentration is varied in the range from 600 to 7000 ppmv. In the concentration range studied, the elimination capacity increased proportionately with an increase in feed concentration. However, the elimination capacity decreased significantly at flow rates greater than 20 dm(3) min(-1) owing to insufficient residence time. The maximum elimination capacity observed was 185 g ethanol h(-1) m(-3) of reactor volume. Critical ethanol loading, defined as the maximum loading to achieve greater than 99% elimination at various residence times have been determined. These data are extremely useful in designing bioreactors for large scale applications.