A decomposition mechanism of H2O2 by triphenylphosphine oxide (TPPO) is presented. TPPO is often incorporated in proton-exchange membrane electrolytes as a moiety to inhibit the H2O2-induced degradation of the membranes. However, it has not been revealed how TPPO decreases the concentration of free H2O2 in the membranes. Following the experimental X-ray structures, the TPPO dimer capturing two H2O2 molecules was used as the calculation model. The vibrational spectrum calculations for various hydration numbers show that this model correctly reproduces the spectral peaks of TPPO capturing H2O2. On the basis of this model, the H2O2 decomposition mechanism by the TPPO dimer was searched. It was consequently found that this reaction proceeds through three steps: (1) Hydrogen transfer from H2O2 to the P=O bond of TPPO, (2) Hydrogen transfer from the -OOH group to the -OH group, and (3) O-O bond formation between O2 groups. The calculated vibrational spectra for the reactants and intermediates indicated that the first and second steps are activated by vibrational excitations. Moreover, the third step giving low barrier heights is considered to proceed through two reaction paths: directly producing the O2 molecule or going through an HOOOH intermediate. Interestingly, this reaction mechanism was found to use the violation of the octet rule for the P=O double bond, resulting in the strong H2O2 binding of TPPO.