Encoding information in the chemical structure of tectons is the pivotal strategy in self-assembly for the realization of targeted supramolecular structures. However, frequently observed polymorphism in supramolecular monolayers provides experimental evidence for a decisive additional influence of environmental parameters, such as solute concentration or type of solvent, on structure selection. While concentration-induced polymorphism is comparatively well understood, the thermodynamical and molecular origins of solvent-induced polymorphism remain elusive. To shed light on this fundamental aspect of self-assembly, we explore the solvent-induced polymorphism of trimesic acid (TMA) monolayers on graphite as prototypical