pH and oxidation reduction potential (ORP) values of waters in original positions (in-situ) were estimated using analytical data on deep groundwaters and hot spring waters at 67 points in Japan. A predominant redox reaction at each point was thermodynamically analysed on the basis of the Gibbs reaction energy. The results showed that a redox reaction of iron sulphide / sulphate ion was estimated to be predominant at many points, and that in a pH-ORP diagram, most of the in-situ pH and ORP estimates were on an equilibrium curve between pyrite and sulphate ion. It is suggested that the in-situ groundwaters in the Tertiary sedimentary rocks in Japan could be governed with the redox reaction of the iron sulphide / sulphate ion.