In this article, we studied the influence of light exposure on the electrical properties of a pentacene-based transistor. While the charge carrier mobility is found unaffected at 2.10(-2)cm(2) V(-1)s(-1) after 2 hours of constant ultraviolet illumination, the photo-excitation mainly perturbs the charge carrier density in the active region. Irreversible change of the mobility was only observable after 10 hours of light exposure. The corresponding phototransistor performance under UV show a maximal current gain (I-dsillumination/I-dsdark) at Vgs = 0V of 10(3). Moreover, light intensity dependent measurements revealed both a photovoltaic (in turn-on state) and a photoconductive (in turn-off state) effect in the device.