The performance of a thermophilic biofilter (50 °C) was evaluated to treat a mixture of gas-phase benzene, toluene, ethylbenzene, p-xylene, m-xylene, and o-xylene (BTEX), under steady- and transient-state conditions. After acclimation, steady-state total BTEX elimination capacities ranged from 6 to 360 gm−3 h−1, depending on the total BTEX loading rate applied (8–978 gm−3 h−1). CO2 production from BTEX biodegradation was found to be dependent on the total elimination capacity of the biofilter. Stratification in BTEX removal profiles along the biofilter bed height was ascertained to be a strong function of the concentrations of the individual pollutants. Under steady-state conditions, higher removal efficiencies for benzene (often 100%) was noticed in comparison to the alkylbenzene compounds at the same initial load. Furthermore, studies were carried out under shock loading and starvation conditions (transient loads) for periods of 4–6 days, showing the robustness and resilience capacity of the biofilter, as the biofilter immediately recovered its original removal efficiency when high loads were applied after the brief transient conditions.