Two polymorphs of the 2:1 molecular complex of isonicotinamide and oxalic acid have been characterized by combined X-ray charge density and single-crystal neutron diffraction Studies It 100 K. Both polymorphs show strong O center dot center dot center dot H center dot center dot center dot N intermolecular hydrogen bonding between the acid and the pyridine base. As is typical of short, strong hydrogen bonds (SSHBs), the covalent O-H bonds are considerably elongated to 1 161(3) and 1 235(5) angstrom, and the H center dot center dot center dot N interactions ire correspondingly short at 1.398(3) and 1.313(6) angstrom in Forms I and II, respectively. The neutron diffraction data indicate no pronounced H dynamics in the SSHBs, and ill the case of Form 11 the SSHB call be described as quasicentered, In addition to the experimental charge densities, theoretical charge densities have been determined from ab initio calculations within the full periodic environment of the crystalline state. The SSHBs ire found to be covalent in nature according to the topological analysis of the experimental and theoretical charge densities, and application of the source function. Aside from the SSHBs, moderate N center dot center dot center dot H center dot center dot center dot O and weak C-H center dot center dot center dot O interactions are also present in the molecular complexes, for which hydrogen bond energies are estimated From energy densities and independent ab initio calculations. Finally in attempt is made to evaluate the intermolecular interactions governing the manifestation of polymorphism in this compound.