Carbon dioxide served as a proxy for biodegradation in the biofiltration of multiple organic solvents in a batch biofilter. The solvents were injected into the batch biofilter system and were biodegraded with the biomass immobilized on a column of fibrous media; the immobilized biomass was acclimated to ethanol in a continuous biofilter. Time profiles of CO 2 concentration were obtained for substrate utilization and for biomass endogenous respiration. The CO 2 and substrate concentration profiles provided insight for the biodegradation of multiple substrates in a biofilter. Experimental results indicate that the biomass utilized the solvents simultaneously but at different rates. The utilization rates were in the order: acetaldehyde > ethanol > propanol > isopropanol, acetone > methanol > butanol. Isopropanol was transformed to acetone, which was then utilized and converted to CO 2 , and propanol was also utilized through the formation of an intermediate. The biodegradation of ethanol was inhibited by acetaldehyde. Total CO 2 production was contributed by the endogenous respiration of the biomass as well as by the biodegradation of all the substrates present to the biomass, and the total CO 2 rate might not reach its maximum potential.