any of a class of diagrams that illustrate the fields of stability of mineral or chemical species in terms of the activity of hydrogen ions (pH) and the activity of electrons (Eh). Consequently, the reactions illustrated on Eh–pH diagrams involve either proton transfer (e.g., hydrolysis) or electron transfer (oxidation or reduction) or both. In natural environments, pH values extend from 1 to 9.5, and Eh values from -500 to +800 millivolts. Rarely are temperatures and pressures other than those normally encountered on the Earth's surface considered.

The area on an Eh–pH diagram that represents the range of these variables within which a particular mineral is stable is called the stability field of that mineral. Such a representation enables a geochemist to determine whether a mineral is in equilibrium with its surroundings or subject to chemical transformation. See also geochemical facies.