The biodegradation reaction of volatile organic compounds (VOCs) in a sealed reactor was studied using toluene and octane as model VOCs. Pseudomonas putida mt-2 and Pseudomonas oleovorans, which are known to degrade toluene and octane, respectively, were selected as model strains for the biodegradation. In addition, benzyl alcohol and octanol, which are the first degradation products of toluene by P. putida mt-2 and of octane by P. oleovorans were selected as target substrates. The octane-degrading strain, P. oleovorans, could not degrade toluene, and in the presence of toluene its ability to degrade octane was reduced due to the toxicity of toluene. The toluene-degrading strain, P. putida mt-2, could also degrade octanol, but in a mixture of toluene and octanol, it died due to the toxicity of the mixture. Except when analyzing the degradation mechanism of a single substrate, it is found to be important to take into account the effect of multiple substrates on the microorganisms concerned.