Molecules adsorbed on rough metallic surfaces give very strong infrared absorption. The infrared spectroscopy using this effect, surface-enhanced infrared absorption spectroscopy (SEIRAS), has successfully been applied to in situ, time-resolved monitoring of reactions at the electrochemical interface and provided several new insights into electrochemical surface science. Mechanistic studies on the fundamental electrocatalytic reactions relevant to fuel cells are focused in this review. The reactions treated in this review are hydrogen evolution reactions on Pt and Ag, and electrochemical oxidation of methanol, formaldehyde, and formic acid on Pt, through which it is highlighted that time-resolved analysis of reactions is quite essential for understanding reaction mechanisms at the molecular scale.