Ultraviolet, infrared, and nuclear magnetic resonance spectra, and ionization constants, have been determined for 4-nitraminopyridine, 4-(methylnitramino)-pyridine, 1,4-dihydro-1-methyl-4-nitriminopyridine, 4-hydrazinopyridine, 1,4-dihydro-1-methylpyridin-4-one hydrazone, and their ions. 4-Nitraminopyridine exists in the nitrimine tautomeric form (II) in the solid state, in water (in c. 99%), ethanol, dimethyl sulphoxide, and acetonitrile, but in dioxan it is mostly (c. 94%) in the nitramino form (I). 4-Hydrazinopyridine exists predominantly in the hydrazine form. Infrared spectra characteristic of pyridinium ions are shown by the cation of 4-nitraminopyridine (X) and the dication of 4-hydrazinopyridine (XIV); in both the ring nitrogen carries a positive charge. The infrared spectrum of the monocation of 4-hydrazinopyridine is not that of a pyridinium ion, and shows 4-pyridonoid features; the positive charge resides essentially on the 4-nitrogen atom (XIII), as in the uninfstituted 4-aminopyridine cation. Cation formation moves the a- and β-proton magnetic resonances of both 4-(methylnitramino)pyridine and 1,4-dihydro-1-methyl-4-nitriminopyridine to infstantially lower 7 values. Monocation formation reduces the T value of the β-proton resonance of 4-hydrazinopyridine by a relatively small amount, and changes that of the α-proton only slightly (upward). Formation of the dication of 4-hydrazino-pyridine (XIV) lowers both T values infstantially.