Clarification is reported of the mechanism for the appearance of hidden negative differential resistance, HNDR, in the oxidation of formic acid using surface-enhanced infrared absorption spectroscopy, SEIRAS, simultaneously with electrochemical measurements. The HNDR is necessary for the appearance of potential oscillation. After holding the potential at 0.7 V for 300 s, the voltammogram exhibits a long monotone current decrease, which means NDR and is named LCD, in a wide potential range between 0.5 and 0.8 V. In the LCD potential range, the voltammogram is reversible with regard to potential sweep directions and there is neither adsorbed CO nor adsorbed hydroxide. The latter information is obtained by the voltammogram in the absence of formic acid and by the change in the baseline of the SEIRA spectra. We hence think that the LCD appears due to the adsorption of water, which increases in residence time on the surface with potential due to the water-surface interaction. Based on this idea, we have been able to mathematically reproduce the experimentally observed oscillation pattern. Using a pseudo-stationary voltammogram and simultaneously measured SEIRA spectra, we conclude that the HNDR appears due to the formation of adsorbed CO, which hides the LCD between 0.5 and 0.7 V. (C) 2014 Elsevier Ltd. All rights reserved.