A new method for studying surface reaction of LaNi5 absorbing alloy in KOH solution (pH 12) was established. It is based on tip-substrate voltammetry of scanning electrochemical microscopy (SECM) where the tip faradic current is recorded while scanning the substrate potential. The Pt electrode is selected as tip electrode, and the Pt oxide formation-reduction is used as a pH-dependent reaction while the tip potential is held at a constant value. As substrate surface reactions proceed, the pH of solution can be changed, and then the tip faradic current is recorded. The mechanism of discharge process of LaNi5 alloy was analyzed by comparing the tip current (I-tip) versus substrate potential (E-sub) curve, which reflects the exchange of H+ or OH-between the alloy surface and the solution, with the substrate current (I-sub) versus substrate potential (E-sub) curve, which reflects the exchange of electron on the LaNi5 alloy surface. The results showed that the OH- adsorption process is occurred before the electron transfer process during discharge process, and the adsorptive OH- helps the oxidation of adsorbed hydrogen atom on the alloy surface. A quantitative assessment for the maximum changes of pH during discharge process is also proposed, and the variation as large as 2.65 pH unit was detected. (c) 2006 Elsevier Ltd. All rights reserved.