Study of 6-azaCyt, 6-azaC, a number of their derivatives and related compounds was conducted by IR and NMR spectrocsopies. Doublet splitting of amino group signal in NMR spectra of 6-azaCyt (unlike the cases of canonical base Cyt, nucleosides C and dC under the same experimental conditions) indicates nonequivalency of amino protons caused by greater asymmetry of electron structure which increases on ring substitutions at the 1 and 5 positions and increased barrier of amino group rotation. The dow field component of the doublet is only responsive to the 5-methyl substitution, as probably related to the involvement one of amino protons into an intramolecular H-bond with the N3 atom. Inverse (as compared to Cyt) specificity of interactions of 6-zaCyt with the amino acid carboxylic group and carboxylate-ion in anhydrous DMSO has been shown. The spectroscopy data support the conclusion that investigated compounds with nonsubstituted amino group exist as keto-amino tautomers in DMSO and solid state.