The optimized molecular structures, vibrational frequencies and corresponding vibrational assignments of conformations of 2-hydroxy-3-nitropyridine and 3-hydroxy-2-nitropyridine molecules have been investigated using ab initio Hartree-Fock (HF) and density functional theory (B3LYP) methods with 6-311++G(d,p) basis set. The comparison of the experimental and calculated spectra of the molecules have shown that they exist in two conformations with the two O-H bond angles (110 degrees and 250 degrees) respective to the C-O bond in the ground state and their energy curves having two minimums have been drawn. (C) 2007 Elsevier B.V. All rights reserved.