The activation and utilization of the greenhouse gas CO2 is of great interest for the energy transition as a fossil-free carbon source for mitigating climate change. CO2 hydrogenation via the reverse water-gas shift reaction (RWGSR) converts CO2 to CO, a crucial component of syngas, enabling further transformation by means of the Fischer-Tropsch process. In this study, we unravel the detailed mechanism of the RWGSR on low-loaded Au/CeO2 catalysts using IR modulation excitation spectroscopy (MES), by periodically modulating the concentration of the reactants, followed by phase-sensitive detection (PSD). Applying such a MES-PSD approach to Au/CeO2 catalysts during RWGSR gives direct spectroscopic evidence for the active role of gold hydride, bidentate carbonate and hydroxyl species in the reaction mechanism, while disproving the participation of other species such as formate. Our results highlight the potential of modulation excitation spectroscopy combined with phase-sensitive detection to provide new mechanistic insight into catalytic reactions not accessible by steady-state techniques, including a profound understanding of the sequence of reaction steps.