FT-IR and Raman spectra of 1(2H)-phthalazinone have been experimentally recorded in the region 400-4000 cm(-1) and 400-3200 cm(-1), respectively. The molecular geometry and vibrational frequencies of 1(2H)-phthalazinone in the ground state have been calculated by using ab initio Hartree-Fock (HF) and density functional theory (DFT/B3LYP) calculations with the 6-311++G(d,p) basis set. The obtained optimized geometric parameters (bond lengths and bond angles) and vibrational frequencies were in very good agreement with the experimental data. The comparison of the observed fundamental vibrational frequencies and calculated results for the fundamental vibrational frequencies of 1(2H)-phthalazinone exhibits that the scaled B3LYP method is superior compared to the scaled HF method. Published by Elsevier B.V.