We use molecular modeling and the simulation of X-ray diffraction patterns to determine the molecular packing of a thiophene-based polymer showing exceptionally high field-effect mobilities (up to 1 cm2 V−1 s−1). We focus on the organization of the polymer chains in lamellae and the orientation of these crystalline domains with respect to the substrate in thin films. The analysis is supported by XRD and NEXAFS experiments and is complemented by calculating intermolecular transfer integrals, which govern the charge mobility.