The goal of this study is to determine the most stable tautomeric forms, and their ground state conformers of 4'-nitroazobenzene-2,4-diol and 4-methyl-4'-nitroazobenzene-2,6-diol compounds. The calculations have shown that the most stable tautomeric forms of the compounds are hydrazo form for 4'-nitroazobenzene-2,4-diol and azo form for 4-methyl-4'-nitroazobenzene-2,6-diol. Besides, the vibrational frequencies, H-1 and C-13 NMR shifts, frontier molecular orbital's energies for the tautomeric forms of the compounds calculated by using density functional theory-B3LYP method with 6-311G(d) basis set were interpreted. All the assignments of the theoretical frequencies were identified by potential energy distribution (PED) analysis. Generally, theoretical spectral results were seen to be in a good agreement with the corresponding experimental data.