This chapter introduces concepts based in recent literature. Experimental measurements show that electrons at the valence band at the top of an insulatorInsulator surfaces can be in equilibrium with near surface reactions in solution. The example is diamondDiamond. Two effects are demonstrated. For the first, oxygen and carbon dioxide in the ambient atmosphere oxidize the surface of diamond to form holes (h+dia), and bicarbonate anions. In this double-layer system the holes in the surface of the insulator are the charge carriers for p-typeP-type conduction laterally across the surface region. Second, the electrochemical potential of electrons in solution with oxygen present is determined by its four-electron reduction potential for forming H2O and hasPH dependence pH dependence determined from the Nernst equationNernst equation. Depending on pH, electrons can transfer from or to the diamondDiamond surface until the chemical potential of electrons in the surface matches that in the solution so that equilibrium is established with the oxygen containing solution. The same mechanism can make band gap state occupations depend on pH, affecting optical properties of other insulatorsInsulator.