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Interfacial structure is critical for optimizing the electrical properties of organic field‐effect transistors. In this study, the interfacial structures of 6,13‐bis(triisopropylsilylethynyl)pentacene (TIPS‐pentacene)/polymer blends are nondestructively determined by the complementary neutron and X‐ray reflectivity. The TIPS‐pentacene/deuterated poly(methylmethacrylate) (d‐PMMA) blends exhibit a vertically phase‐separated structure with a molecularly sharp interface (interfacial roughness ≈5 Å), whereas the TIPS‐pentacene/d‐polystyrene (d‐PS) blend intermix near the interface. Ultrahigh molecular weight d‐PMMA leads to the formation of surface‐segregated hexagonal spherulites of TIPS‐pentacene owing to the thermodynamic factors (e.g., surface/interface energy, polarity, and viscosity) of the blending materials. The well‐developed hexagonal spherulites of TIPS‐pentacene on molecularly sharp d‐PMMA interface result in higher field‐effect mobility as compared to the dendritic crystals from d‐PS blends because of the higher perfectness, coverage, and interfacial roughness of the TIPS‐pentacene crystals. The approach used in this study facilitates the understanding of the charge transport mechanism at the phase‐separated interfaces in soluble acene/polymer blends. [ABSTRACT FROM AUTHOR]