Abstract: Equilibrium geometries of cis-HOOOH and trans-HOOOH have been investigated using the density functional theory (DFT), complete active space self-consistent-field (CASSCF), and coupled cluster with single and double replacement (CCSD) approaches. The harmonic vibrational frequencies on the optimized geometries were calculated using the DFT theory. The potential energy curve of the isomerization between the trans-HOOOH and cis-HOOOH was obtained by DFT calculations. Time-dependent density functional theory (TD-DFT) and complete active space perturbation theory of second order (CASPT2) calculations have been performed to obtain the vertical excitation energies of selected low-lying singlet and triplet excited states. Computed results show that: (1) trans-isomer is more stable than cis-isomer; (2) there are two pathways of the conversion between the trans-HOOOH and cis-HOOOH; (3) the vertical excitation energies of the lowest singlet and triplet excited states in trans-HOOOH are lower than those in cis-isomer; (4) in the singlet excited states, 21A state in trans-HOOOH and 21A′′ state in cis-HOOOH have the longest lifetimes of 2.07×10−5 s and 1.44×10−5 s with the excitation energies of 165.52 and 167.43 nm, respectively.