Hydrogen-bonded pigments are extremely stable, generally non-toxic materials which have recently found applications in organic electronics due to the observation of promising charge carrier mobilities in organic field-effect transistors (OFETs).1–6 Of the hydrogen-bonded pigments studied to date, epindolidione (EPI), a hydrogen-bonded derivative of the well-known organic semiconductor tetracene, is one of the most promising, with hole mobilities of up to 1.5 cm2 V–1 s–1 observed in thin film transistors.3 As for any molecular material, the physical properties are strongly influenced by the arrangement of the molecules in the solid state (i.e. the crystal structure).7 For charge transport in thin films, the molecular arrangement in the vicinity of the substrate is particularly important,8 and it has also been shown that new polymorphs, known as surface-mediated phases, can appear in this region.9 Structural characterization of thin films of materials to be used in organic electronic devices is therefore crucial to the understanding of their behavior.