Ambipolar thin-film transistors based on a series of air-stable, solution-processed blends of an n-type polymer poly(benzobisimidazobenzophenanthroline) (BBL) and a p-type small molecule, copper phthalocyanine (CuPc) are demonstrated, where all fabrication and measurements are performed under ambient conditions. The hole mobilities are in the range of 6.0 × 10–6 to 2.0 × 10–4 cm2 V–1 s–1 and electron mobilities are in the range of 2.0 × 10–6 to 3.0 × 10–5 cm2 V–1 s–1, depending on the blend composition. UV-vis spectroscopy and electron diffraction show crystallization of CuPc in the metastable α-crystal form within the semicrystalline BBL matrix. These CuPc domains develop into elongated ribbon-like crystalline nanostructures when the blend films are processed in methanol, but not when they are processed in water. On methylene chloride vapor annealing of the blend films, a phase transformation of CuPc from the α-form to the β-form is observed, as shown by optical absorption spectroscopy and electron diffraction. Ambipolar charge transport is only observed in the blend films where CuPc crystallized in the elongated ribbon-like nanostructures (α-form). Ambipolar behavior is not observed with CuPc in the β-polymorph. Unipolar hole mobilities as high as 2.0 × 10–3 cm2 V–1 s–1 are observed in these solution-processed blend field-effect transistors (FETs) on prolonged treatment in methanol, comparable to previously reported hole mobilities in thermally evaporated CuPc FETs. These results show that ambipolar charge transport and carrier mobilities in multicomponent organic semiconductors are intricately related to the phase-separated nanoscale and crystalline morphology.