The electrodes, particularly the Pt(100) plane. Focusing on the analysis of the voltammetric profiles, the effects of the experimental conditions (formic acid concentration, upper and lower potential limits of the sweep, sweep rate, solution pH etc.) were examined extensively. The product survey was performed using the newly developed on-line differential electrochemical mass spectrometry (DEMS). The following results were obtained. (1) The surface species X(1) (linearly bonded CO) formed mainly in the hydrogen region and retarded the reaction strongly up to ca. 0.8 V in the positive-going scan. When the X(1) coverage is lower, the reaction occurred with a peak P-1 at ca. 0.4 V (process I). (2) After the removal of X(1) at ca. 0.8 V (peak P-2), no reaction occurred up to a potential of 1.0 V, suggesting the presence of retarding process. (3) When the potential was reversed at 1.0 V, the retarding process remained beyond 0.8 V in the negative-going scan. The retarding action decreased exponentially with the negative potential shift, and correspondingly the reaction was accelerated. (4) The reaction continued to occur with a peak P-3 at ca. 0.6 V (process II). With further negative polarization, the reaction ceased beyond 0.3 V with a shoulder corresponding to P-1 (process I). (5) Another retarding process was observed in the potential region between P-1 and P-3. The species X(2) responsible was electrochemically inactive. The behavior of X(1) and X(2), processes I and II, and the retarding process on Pt(100), together with the DEMS results, were examined in detail.