Interactions of toluene and p-xylene in air treatment biofilters packed with an inert filter media were studied. The effect of the inlet load of toluene, p-xylene and mixtures of both compounds on the biodegradation rate was analyzed in three lab-scale biofilters. A maximum elimination capacity (EC) of 26.5 and 40.3 gCm(-3)h(-1) for an inlet load (IL) of 65.6 and 57.8 gCm(-3)h(-1) was obtained for p-xylene and toluene biofilters, respectively. Inhibition of p-xylene biodegradation by the presence of toluene took place when the mixture was treated, whereas the presence of p-xylene had an enhancing effect on the toluene removal efficiency. Specific growth rates (μ) from 0.019 to 0.068 h(-1) were calculated in the mixed biofilter, where the highest values were similar to mixtures with lower p-xylene levels (IL(p-Xyl) 8.84 ± 0.29 gCm(-3)h(-1)). Michaelis-Menten and Haldane type models were fitted to experimental EC for p-xylene and toluene biofilters, respectively.