An experiment for five stages of a biofilter-run was performed to investigate the effect of hydrophilic ethanol and hydrophobic toluene on the biodegradation of hydrophobic toluene and hydrophilic ethanol, respectively, when waste-air containing toluene and ethanol was treated by a biofilter. Removal efficiencies of toluene and ethanol began to decrease when inlet load surpassed 90 g/m3/h and 100 g/m3/h consistent with maximum elimination capacities of toluene and ethanol, respectively. At the end of the biofilter-run, removal efficiencies for toluene and ethanol were decreased and maintained at 65% and 40%, respectively. The concentration of toluene at 1st sampling port was raised by factor of two in the 3rd stage of the biofilter run when the inlet load of ethanol co-feed was increased by 1.5 times, while the process conditions of toluene were maintained the same as those of the 2nd stage of biofilter-run. According to the result of Mohseni and Allen, it may be interpreted that removal efficiency of hydrophobic toluene was affected by the presence of hydrophilic ethanol when high load of hydrophobic toluene was applied like that of the 1st sampling port of the biofilter. However it was not the case when a low load of hydrophobic toluene was applied like those of the 2nd, 3rd and 4th sampling ports since hydrophobicity of toluene is much less that of α-pinene. Thus, it may be suggested that biodegradation of hydrophobic VOC was interfered by hydrophilic VOC dissolved in the biolayer and the degree of interference was proportional to the inlet load of hydrophobic VOC as well as that of hydrophilic VOC and was inversely proportional to the solubility of hydrophobic VOC. However, it was inferred that the existence of hydrophobic toluene from waste-air can hardly inversely hinder the removal of hydrophilic ethanol in the biofilter when timeevolutions of hydrophilic ethanol concentrations of this experiment were compared with those of the previous experiment of biofilter to treat waste-air containing ethanol only.