We report the preparation of high-quality, solvent-free [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) large single crystals (size up to 0.5 mm) by slow drying of a chlorobenzene solution at room temperature. The monoclinic structure containing four PCBM molecules per unit cell was successfully solved (R-factor = 0.0512) via micro-focused X-ray diffraction and employed as a reliable experimental model for further molecular dynamics simulations. We find that the first peak of the simulated fullerene–fullerene radial distribution function is centred at 10.05 A, giving a nearest neighbour coordination number of 7.0. The work reported herein provides the structural basis for a fundamental understanding of charge transport in this important functional material that is particularly relevant to organic solar cells.