The presence of adventitious oxygen is inevitable when organic/metal interfaces are formed by evaporation in high vacuum (10(-6) mbar.). In this letter, we highlight the importance of this oxygen for band alignment, and hence, performance, in organic-based devices. The influence of controlled amounts of oxygen on band alignment in benzene/aluminum model cathode interfaces has been studied using ultraviolet photoemission in ultrahigh vacuum. We show that even small amounts of oxygen significantly lower the aluminum work function with concomitant improvement in band alignment. (C) 2000 American Institute of Physics. [S0003-6951(00)02534-1].