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1. The electron and the proton content (measured as electrode potential [Eh] and pH) of an environment characterize this environment in many ways. In this paper the electrode potential and the pH are used as empirical parameters rather than as electrochemical data capable of thermodynamic interpretation. From published and unpublished work by the authors and from the literature, more than 6,200 pairs of characteristics were gathered, covering most types of the aqueous environment as well as the potential milieu of the chief actors in these environments: algae and bacteria. 2. It appears that the Eh-pH limits of biological systems and of the naturally occurring aqueous environment almost coincide. This would indicate that there are few, if any, sterile terrestrial environments caused by limiting Eh-pH characteristics. 3. As it seems unlikely that environments will be found outside the limits outlined in this paper, physico-chemical speculations on the sedimentary environment should be limited by this outline. Substances which do not occur (sulfuric acid, sulfide ion) should not be used in the electrochemical characterization of the environment. 4. The biogenic master reaction in the environment, changing one or both characteristics (Eh-pH), is reductive photosynthesis by algae and by colored bacteria. A photosynthetic mass may raise the pH of a water to 9.4; and in the absence of bivalent cations, to 12.6. 5. The intensity of sulfate reduction depends upon the sulfate content of the water and on the available hydrogen, in both organic and inorganic form. The iron concentration is also important, as iron is the principal acceptor of the $$H_{2}S$$ formed. The highly reactive, black iron Sulfides may be partly oxidized with the formation of the more stable pyrite and marcasite. The reduction of iron from ferric to ferrous state takes place even in surface soil. 6. Denitrification, another biologically important reduction, may be of lesser geochemical influence. 7. Oxidative reactions comprise, apart from nitrification, chiefly the oxidation of $$H_{2}S$$ and $$SH^{-}$$ to sulfur, thiosulfate, sulfite, hydrosulfite, sulfate, and hydrosulfate and the oxidation of ferrous and manganous compounds. In contrast with the reductions, these oxidations are only in part biological. The oxidation of pyrite may give rise to extremely low pH values. Heterotrophic oxidation (respiration) results in the conversion of organic matter into $$CO_{2}$$ and $$H_{2}O$$. 8. Acid formation in peat bogs is caused largely by cation exchange on plant cell walls, chiefly, but not exclusively, on Sphagnum. 9. In sediments the reaction between iron phosphate complexes and $$H_{2}S$$ may liberate the acid $$H_{2}PO_{4}^{-}$$ ion. 10. Certain environments are restricted, others cover almost the maximal area outlined in this paper. A progressive increase in the environmental range, arranged in a series, follows: rain water, mine water, peat bogs, sea water, rivers and lakes, marine sediments, and evaporites, while the geothermal environment shows the maximal area. 11. The potential milieu of the green bacteria is highly restricted. Less restricted is the environment of the iron bacteria, followed by sulfate-reducing bacteria, purple bacteria, and denitrifying bacteria. Thio-bacteria have a very wide potential milieu, and algae are found literally everywhere. 12. The Eh-pH characteristics are determined chiefly by photosynthesis, by respiration and by oxido-reductive changes in the iron and sulfur systems.