In order to get a better understanding of the nature of the N'-H...N-" system characterizing the intramolecular hydrogen bond in classical proton sponges, crystal structure determinations at room temperature and at 150 K have been undertaken for the picrate and the hexafluorophosphate salts of 1,8-bis(dimethylamino)naphthalene (DMAN). Two polymorphic forms were detected for the picrate. The crystals of the orthorhombic form twinned when they were cooled. The elongated N+-H distances, even at low temperature, are indicative of proton disorder. The polymorphism of DMAN picrate proved crucial in establishing the complementarity between the CPMAS NMR (C-13 and N-15) and crystallography results.