The industrial importance of molecular materials chemistry has promoted great interest in areas such as self-assembled surface coatings, multi-layer formation on solid substrates, crystallization from solutions, crystal morphology and structure predictions, solving structures from powders and control of polymorphism. Improvements in our understanding of the role of intermolecular interactions in driving molecular self-assembly and interfacial processes have led to technological advances-both in controlling the assembly of molecules at the nanometre scale, and in manipulating processes and products in which crystal nucleation and growth are key elements. But there has been relatively little work on molecular-scale engineering at solid-solid interfaces, despite their importance in polymeric composites for structured and electronic applications, in adhesives and in formulated pharmaceutical and agrochemical products. Here we report the use of molecules as tailored adhesives - a molecular 'glue' is selected to bond across an interfacial region and hence stabilize a solid-solid interface. We consider a simple interface occurring in a twinned crystal of saccharin; additive molecules with predictable dimensions and hydrogen-bonding functionality can span the interface. The stabilization is reflected in an enhanced frequency of twin-crystal formation.