Unstable intermediates are indispensable ingredients of chemical reactions, but their identification at solid surfaces has been hampered by the lack of techniques to detect short-lived and fractionally generated species. We succeeded in identifying the intermediate in a thermal decomposition reaction by utilizing the picosecond temperature jump induced by the irradiation of laser pulses and the observation by sum-frequency generation spectroscopy. We verified that the decomposition of formate on the NiO(lll) surface is preceded by the transformation of stable bidentate formate to unstable monodentate formate, which is the reactive intermediate, and two species are found to be in equilibrium before decomposition.