Electrospun nanofibers of two series of binary blends of poly [2-methoxy-5-(2-ethylhexoxy)1,4-phenylenevinylene] (MEH-PPV) with regioregular poly(3-hexylthiophene) (PHT) and with poly(9,9-dioctylfluorene) (PFO) were prepared, and their morphology and optical and electrical properties were characterized. Morphological and photophysical studies showed that the phase-separated domains in MEH-PPV/PHT nanofibers (30-50 nm) are much smaller as compared to blend thin films (100-150 nm), and efficient energy transfer was observed in these blend nanofibers. The MEH-PPV/PFO blend nanofibers had cocontinuous or core-shell structures, and significant energy transfer was absent in these blend nanofibers as compared to bulk thin films. Field-effect transistors based on MEH-PPV/PHT blend nanofibers showed exponential dependence of hole mobility on blend composition. The hole mobility decreased from 1 x 10(-4) cm(2)/(V S) in 20 wt % MEH-PPV blend nanofibers to 5 x 10(-6) cm(2)/(V s) at 70 wt %. If corrected for the reduced channel area of the transistors, the effective hole mobility varies from 5 X 10(-5) to 1 X 10(-3) cm(2)/(V s) and is similar to that of spin-coated blend thin films. Our results demonstrate that electrospun nanofibers of binary blends of conjugated polymers have tunable, composition-dependent, optical, and electronic properties that can be exploited in field-effect transistors.