Three isomeric forms of 1-(pyridylmethyl)-2,2′-biimidazole, A1–A3, have been synthesized and subjected to systematic co-crystallizations with selected hydrogen- and halogen-bond donors in order to explore the impact of electrostatics and geometry on the resulting supramolecular architectures. The solid-state supramolecular behavior of A1–A3 is largely consistent in halogen-bonded co-crystals. Only two types of primary interactions, the N–H⋯N/N⋯H–N homomeric hydrogen-bond interactions responsible for the pairing of biimidazole moieties and the I⋯N(pyridine) halogen bonds responsible for the co-crystal formation and structure extension, are present in these systems. The co-crystallizations with hydrogen-bond donors (carboxylic acids), however, lead to multiple possible structural outcomes because of the presence of the biimidazole–acid N–H⋯OC/N⋯H–O heterosynthon that can compete with biimidazole–biimidazole N–H⋯N/N⋯H–N homosynthon. In addition, the somewhat unpredictable nature of proton transfer makes the hydrogen-bonded co-crystals structurally less consistent than their halogen-bonded counterparts.