Blends of poly(9,9-dioctylfluorene) (PFO) and poly(2-methoxy-5(2'-ethyl-hexyloxy)-1,4-phenylenevinylene) 1,4-phenylenevinylene) (MEH-PPV) were found to phase separate into 40-50 urn crystalline PFO domains and to exhibit efficient white electroluminescence when the composition is below 30 wt.-% MEH-PPV The 5 wt.-% nanocrystalline blends had a luminance of 4000 cd.m(-2), an external quantum efficiency of 3.1%, and a current efficiency of 3:7 cd . A(-1). Transmission electron microscopy, electron diffraction, and atomic force microscopy of blends with higher MEH-PPV content and the two homopolymers showed them to be amorphous. Only orange-red electroluminescence, characteristic of MEH-PPV, was observed from the amorphous blends due to efficient energy transfer from PFO. These results demonstrate that energy transfer processes in binary PFO:MEH-PPV blends and light-emitting devices based on them can be controlled through the morphology and composition.