Direct observation of key intermediates on the catalyst surface is one of the biggest challenges in various photo/electro-catalysis including CO₂ reduction reaction O₂ reduction reaction, etc. For a rational understanding of the reaction mechanism, combined studies encompassing complementary tools such as electrochemical characterization, computational calculations and operando spectroscopies should be conducted. Among various operando spectroscopies, the time-resolved attenuated total reflection-surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) is particularly suitable for the investigation of electrochemical CO₂RR as it allows observation of interfacial processes in real-time owing to its high surface sensitivity. Additionally, time-resolved analysis would reveal the kinetic relevancy of intermediates and their relationship with the reactants and the products based on their dynamic behavior during the reaction. Lastly, a combination of the operando spectroscopic studies with material characterization would enable a correlation of the behavior of the intermediates and the catalyst conditions. Herein, I will introduce in situ time-resolved FT-IR (or ATR mode) spectroscopic techniques to acquire the isotopically labeled products and key intermediates from the photoreduction of carbon dioxide.