The hypophosphite inertness to be oxidized in aqueous solution with concomitant hydrogen evolution, unless in the presence of a catalytic surface, is well known. 1,2 Among the catalytic metals, nickel is the one on which more electrochemical studies concerning the hypophosphite oxidation have been performed. [3][4][5] Although several authors point out that its catalytic activity is strongly dependent on the nickel surface state, there is no agreement on the interpretation of this phenomenon. [5][6][7] In order to understand this dependence it is very important to analyze the effect of the pH solution on the catalytic activity toward the hypophosphite oxidation. Several studies have been carried out in the alkaline, 5,8,9 neutral, 10 and slightly acid 11 pH ranges, but so far no studies are reported in strongly acidic medium.The main reason for the lack of studies in strong acid pH range must be related to the difficulty in controlling the nickel surface state, avoiding the nickel electrodissolution. It is known that the rate of this surface reaction strongly depends upon the nature of the anion electrolyte, which has been interpreted as a consequence of the anion ability to adsorb and form a surface complex promoting the electrodissolution process. 12-14 It is thus expected that the anion electrolyte in a hypophosphite-containing solution in strong acid media affects both the rate of the nickel dissolution and the Ni electrocatalytic activity toward the hypophosphite oxidation. In fact, due to the importance of the hypophosphite adsorption for the hypophosphite oxidation onset on nickel, 8 competitive adsorption of the anion electrolyte may affect the catalytic surface activity.It is thus our aim in this work to establish the experimental conditions regarding the nature of the anion electrolyte which allow surface dissolution to be minimized and catalytic activity toward the hypophosphite oxidation to be maximized in strong acid media. Once the anion electrolyte is chosen, the effect of the solution pH (still in strong acid media) in those electrode reactions is analyzed.The electrochemical study has been undertaken in solutions of 0.1 M H 3 PO 4 , 0.1 M H 2 SO 4 , 0.1 M HClO 4 , and 0.1 M HNO 3 electrolytes containing hypophosphite and in hypophosphite-free solutions. At this pH medium (ca. pH 1) the predominant anion electrolyte should be H 2 PO 4 Ϫ , HSO 4 Ϫ , ClO 4 Ϫ , and NO 3 Ϫ , respectively, and the electroactive species should be present as 90% H 3 PO 2 and 10% H 2 PO 2 Ϫ .Experimental Cyclic voltammetric, current density-time (i-t), and potentialtime (E-t) experiences were carried out in a conventional three-compartment electrochemical cell, provided with a Luggin capillary. A platinum foil was used as a counter electrode. The electrolysis experiments were performed using a one-compartment electrochemical cell. The counter and reference electrodes were a platinum gauze and a saturated calomel electrode (SCE), respectively, which all potentials are referred to on both electrochemical cells. The working ...