A two-stage bioreactor, which consists of a biotrickling filter module and a biofilter module in series, was investigated for the enhanced treatment of a VOC mixture, toluene and methyl ethyl ketone (MEK). Throughout the experiments, the overall inlet loading rate was maintained at approximately $43g/m^3/hr$, but the inlet ratios of the VOCs were modified. The experimental results showed that the different ratios of the VOC mixture resulted in changes of overall removal efficiencies, elimination capacities (ECs) and microbial accumulation on the surface of each packing material. The ratio of inlet toluene to MEK at 50 : 150 was found to be most effective in terms of the overall removal efficiency, because, at this condition, MEK (i.e., the hydrophilic compound) was mostly removed in the biotrickling filter module and the following biofilter module was used to remove toluene. It was also found that when the inlet loading rate of the VOC mixture was serially increased stepwise within short-term periods, the ECs for toluene dropped significantly but the ECs for MEK increased at the ratio of the VOC mixture. These results implied that substrate interaction and/or substrate preferable utilization might have an effect on the biological removal of each compound in the two-stage bioreactor; therefore, the bioreactor should be operated in the condition where the substrate interaction could be minimized in order to maximize overall performance of the two-stage bioreactor.