The effect of chloride anions in electrolyte on formic acid oxidation (FAO) at Pd electrode is investigated by cyclic voltammetry and in situ surface-enhanced infrared absorption spectroscopy with attenuated total reflection (ATR-SEIRAS). Electrochemical measurement indicates that 1-2 x 10(-5) M Cl- in 0.1 M HClO4 + 0.1 M HCOOH solution leads to a significant decrease of formic acid oxidation current. Molecular level results from the ATR-SEIRAS indicate that such a small amount of chloride suppresses the adsorption of ClO4- and HCO3-, increased the CO coverage and somehow stabilized the bidentate formate (HCOOB) species over the main oxidation potentials. The blocking of active Pd surface sites by Cl- and COad is mainly responsible for the oxidation current density decay, and the inconformity of upsilon(HCOOB) band intensity and oxidation current implies that HCOOB is probably not the active intermediate in the direct pathway of formic acid oxidation. (C) 2016 Elsevier B.V. All rights reserved.