Operation of high-surface-area platinum-black electrodes in hot concentrated phosphoric acid produces a loss of surface area with time, giving consequent loss of performance. The loss of surface area is the result of sintering produced by surface migration of platinum atoms on the individual crystallites to sites of lower surface energy. As the sintering process is a surface phenomenon, potential changes cause changes in the surface tensions of the metal crystallites giving rise to variations in sintering rate. The rate of sintering is greatest at low potentials and smallest at high. Carbon monoxide and phosphoric acid impurities adsorbed on the surfaces of the platinum crystallites retard the sintering at low potentials. Similar adsorption-induced changes in physical properties of materials (Rebinder effects) have been observed with ionic solids, but in only a few instances is this effect known to operate on metals. The presence of a strongly adsorbed molecule on the metal surface alters the inter-atomic bonding between the surface metal atoms and illustrates the dependence of the sintering mechanism on the mobility of the surface atoms. Changes in crystallite morphology accompany the sintering process and are considered to be the result of a surface rearrangement of ad-atoms or clumps with elimination of the [100] faces. The activation energy for the sintering process is 25 ± 2 kcal/mole at 0·5 V and is not significantly changed by adsorbed carbon monoxide on the platinum.