The optimized molecular structures, vibrational frequencies and corresponding vibrational assignments of 10 tautomeric forms of a heterocyclic disazo dye [5-(3-methyl-4-phenylazo-1H-pyrazole-5-ylazo) barbituric acid] have been calculated using ab initio Hartree-Fock (HF) and density functional theory (B3LYP) methods with 6-31G(d,p) basis set level. Calculations have shown that the most probably preferential form of the heterocyclic disazo barbituric acid dye in the ground state is azo-hydrazo-keto form corresponding to the experimental results. (C) 2008 Elsevier B.V. All rights reserved.