In this article, recent studies on the electron transfer reaction and the proton transfer reaction in supercritical water were reviewed. Back electron transfer rate and the successive vibrational relaxation of p-nitroaniline dissolved in supercritical water were monitored by the ultrafast transient absorption spectroscopy. It was described how the changes of the dielectric constant and the hydrogen-bonding ability of water affect these phenomena. The proton transfer dynamics of 5-cynano-2-naphthol was studied by the time-resolved fluorescence spectroscopy. The reaction process was also strongly dependent on the hydrogen-bonding ability between water molecules.