The ground-state structure and electronic and vibrational spectra of octaethylporphyrin diacid (H 4OEP 2+) have been studied with the density functional theory. The geometrical parameters computed with B3LYP, PBE1PBE and mPW1PW91 functionals and 6-31G* basis sets are well consistent with the experimental values. Electronic absorption spectrum of H 4OEP 2+ has been studied with the time-dependent DFT method, and the calculated excitation energies and oscillator strengths are compared with the experimental results. The Raman and IR spectra of H 4OEP 2+ and the Raman spectrum of its N-deuterated analogue (D 4OEP 2+) were measured. The observed Raman and IR bands have been assigned based on the frequency calculations at the B3LYP/6-31G* level of theory.