Thin film transistors based on regioregular poly(3-hexylthiophene) (PHT) and poly(3-decylthiophene) (PDT) and their binary blends were investigated and the blend composition dependence of the charge carrier mobility was determined. The field-effect mobility of holes (mu(h)) in a series of 10 binary blends of poly(3-hexylthiophene) (mu(h) = 0.02 cm(2)/(V s)) and poly(3-decylthiophene) (mu(h) = 8 x 10(-5) cm(2)/(V s)) was found to be relatively high (2 x 10(-3) cm(2)/(V s)) and constant over a broad composition range (5-80 wt % PDT). Above 80% PDT, the hole mobility decreased exponentially with composition. Atomic force microscopy of the homopolymers and blends confirmed the absence of phase separation in the blends. A similar interlayer d spacing that is intermediate between those of the homopolymers is implied by the constant hole mobility in the single-phase crystalline blends of 5-80 wt % PDT. These results demonstrate that blends of conjugated polymers are alloy semiconductors in which high and tunable charge carrier mobility can be realized.