This study investigates the protonation pathway in CO₂ photoreduction on TiO₂ nanoparticles using kinetic isotope effects (KIE). The research challenges the long-held assumption that electron-initiated activation is the primary mechanism, instead supporting a protonation pathway. By employing isotopically labeled H₂O/D₂O and in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), the study observes H⁺/D⁺-protonated intermediates and captures an inverse decay KIE. These findings suggest that the protonation pathway involves the formation of O=C=O-H⁺/D⁺ intermediates, which undergo hybridization changes from sp² to sp³ during electron transfer. The results provide new insights into the CO₂ uptake mechanism in semiconductor photocatalysts and highlight the importance of protonation in CO₂ photoreduction. The study also demonstrates that the protonation pathway is not dependent on the presence of a water solvent, as water vapor can achieve similar results. The findings have significant implications for the development of more efficient and sustainable photocatalytic CO₂ reduction technologies.This study investigates the protonation pathway in CO₂ photoreduction on TiO₂ nanoparticles using kinetic isotope effects (KIE). The research challenges the long-held assumption that electron-initiated activation is the primary mechanism, instead supporting a protonation pathway. By employing isotopically labeled H₂O/D₂O and in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), the study observes H⁺/D⁺-protonated intermediates and captures an inverse decay KIE. These findings suggest that the protonation pathway involves the formation of O=C=O-H⁺/D⁺ intermediates, which undergo hybridization changes from sp² to sp³ during electron transfer. The results provide new insights into the CO₂ uptake mechanism in semiconductor photocatalysts and highlight the importance of protonation in CO₂ photoreduction. The study also demonstrates that the protonation pathway is not dependent on the presence of a water solvent, as water vapor can achieve similar results. The findings have significant implications for the development of more efficient and sustainable photocatalytic CO₂ reduction technologies.