In voltammetry and related techniques, a substance that undergoes a change of oxidation state, or the breaking or formation of chemical bonds, in a charge-transfer step. If an electroactive substance B is formed, in the solution or electrode, by a chemical reaction from another substance C, the substance C should be called the precursor of B. In potentiometry with ion-selective electrodes, a material containing, or in ion-exchange equilibrium with, the sensed ion. The electroactive substance is often incorporated in an inert matrix such as poly(vinyl chloride) or silicone rubber.