Undoped, high-quality diamond is, under almost all circumstances, one of the best insulators known. However, diamond covered with chemically bound hydrogen shows a pronounced conductivity when exposed to air. This conductivity arises from positive-charge carriers (holes) and is confined to a narrow near-surface region. Although several explanations have been proposed, none has received wide acceptance, and the mechanism remains controversial. Here, we report the interactions of hydrogen-terminated, macroscopic diamonds and diamond powders with aqueous solutions of controlled pH and oxygen concentration. We show that electrons transfer between the diamond and an electrochemical reduction/oxidation couple involving oxygen. This charge transfer is responsible for the surface conductivity and also influences contact angles and zeta potentials. The effect is not confined to diamond and may play a previously unrecognized role in other disparate systems.