Electroluminescent devices have been developed recently that are based on new materials such as porous silicon and semiconducting polymers(2,3). By taking advantage of developments in the preparation and characterization of direct-gap semiconductor nanocrystals(4-6), and of electroluminescent polymers’, we have now constructed a hybrid organic/inorganic electroluminescent device. Light emission arises from the recombination of holes injected into a layer of semiconducting p-paraphenylene vinylene (PPV)(8-10) with electrons injected into a multilayer film of cadmium selenide nanocrystals. Close matching of the emitting layer of nanocrystals with the work function of the metal contact leads to an operating voltage(11) of only 4 V. At low voltages emission from the CdSe layer occurs. Because of the quantum size effect(19-24) the colour of this emission can be varied from red to yellow by changing the nanocrystal size. At higher voltages green emission from the polymer layer predominates. Thus this device has a degree of voltage tunability of colour.