Abstract The surge in anthropogenic CO2 emissions from fossil fuel dependence demands innovative solutions, Womens tights such as artificial photosynthesis, to convert CO2 into value-added products.Unraveling the CO2 photoreduction mechanism at the molecular level is vital for developing high-performance photocatalysts.Here we show kinetic isotope effect evidence for the contested protonation pathway for CO2 New Barrels photoreduction on TiO2 nanoparticles, which challenges the long-held assumption of electron-initiated activation.Employing isotopically labeled H2O/D2O and in-situ diffuse reflectance infrared Fourier transform spectroscopy, we observe H+/D+-protonated intermediates on TiO2 nanoparticles and capture their inverse decay kinetic isotope effect.Our findings significantly broaden our understanding of the CO2 uptake mechanism in semiconductor photocatalysts.