Interconversion between dioxygen (O-2) and hydrogen peroxide (H2O2) has attracted much interest because of the growing importance of H2O2 as an energy source. There are many reports on O-2 conversions to H2O2; however, no example has been reported on O-2/H2O2 interconversion. Herein, we describe successful achievement of a reversible O-2/H2O2 conversion based on an N21,N23-dimethylated saddle-distorted porphyrin and the corresponding two-electron-reduced porphyrin (isophlorin) for the first time. The isophlorin could react with O-2 to afford the corresponding porphyrin and H2O2; conversely, the porphyrin also reacted with excess H2O2 to reproduce the corresponding isophlorin and O-2. The isophlorin-O-2/porphyrin-H2O2 interconversion was repeatedly proceeded by alternate bubbling of Ar or O-2, although no reversible conversion was observed in the case of an N21,N22-dimethylated porphyrin as a structural isomer. Such a drastic change of the reversibility was derived from the directions of inner NH protons in hydrogen-bond formation of the isophlorin core with O-2 as well as those of the lone pairs of the inner nitrogen atoms of the porphyrin core to form hydrogen bonds with H2O2. The intriguing isophlorin-O-2/porphyrin-H2O2 interconversion was accomplished by introducing methyl groups at the inner nitrogen atoms to minimize the difference of the Gibbs free energy between isophlorin-O-2/porphyrin-H2O2 states and the Gibbs activation energy of the interconversion. On the basis of the kinetic and thermodynamic analysis on the isophlorin-O-2/porphyrin-H2O2 interconversion using H-1 NMR and UV-vis spectroscopies and DFT calculations, we propose the formation of a two-point hydrogen-bonding adduct between the N21,N23-dimethylated porphyrin and H2O2 as an intermediate.