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The deuterated molecular complexes of isonicotinamide with oxalic acid crystallise in two polymorphs, which are found to be distinct from the two polymorphs of the hydrogenous complexes previously reported. This phenomenon is known as isotopomeric polymorphism, is rarely observed in molecular materials and in particular the presence of multiple polymorphic forms of each isotopic material observed here appears to be unprecedented. The four polymorphic forms are found to exhibit different degrees of hydron transfer. Unlike the hydrogenous forms, the deuterated polymorphs do not show short, strong hydrogen bonding between the acid and the pyridine base. Periodic electronic structure calculations establish an energy scale for the polymorphism in these isotopomeric polymorphs.