Bulk-heterojunction engineering is demonstrated as an approach to producing ambipolar organic light-emitting field-effect transistors with tunable electrical and optoelectronic characteristics. The electron and hole mobilities, as well as the electrolu-minescence intensity, can be tuned over a large range by changing the composition of a bimolecular mixture consisting of alpha-quinqucthiophene and N,N'-ditridecylperylene-3,4,9,10-tetracarboxylic-diimide. Time-resolved photoluminescence spectroscopy reveals that the phase segregation of the two molecules in the bulk heterojunction and their electronic interaction determine the optoelectronic properties of the devices. The results presented show that the bulk-heterojunction approach, which is widely used in organic photovoltaic cells, can be successfully employed to select and tailor the functionality of field-effect devices, including ambipolar charge transport and light emission.