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Supported-metal (Au, Pt) ceria-based catalysts are considered as promising candidates for the water–gas shift (WGS) reaction at low temperatures. Two main mechanisms have been proposed in the literature, the redox and associative mechanisms. A key step in both mechanisms has been considered to be the cleavage of O–H bonds. In this mechanistic study, the role of surface and bulk oxygen species involved in the WGS reaction over ceria-supported gold catalysts (Au/CeO₂) was elucidated directly using operando Raman spectroscopy combined with isotope labeling and supported by DFT+U calculations. Exposure of Au/CeO₂ to pure H₂¹⁸O results in a complete replacement of surface ¹⁶O ions by ¹⁸O ions as rationalized by dissociative adsorption of H₂¹⁸O in the presence of a surface oxygen vacancy and a subsequent backward reaction restoring lattice oxygen as ¹⁸O and releasing H₂¹⁶O. This reaction pathway is accessible even in the absence of CO. Exposure to reaction conditions leads to (i) a complete disappearance of the Ce–O surface modes due to hydroxyl formation, (ii) a Raman F₂g redshift due to reduction of the ceria subsurface, leading to a change in stoichiometry from CeO₁.₉₄₇₋ₓ (in argon) to CeO₁.₈₇₃₋ₓ (in CO/H₂¹⁶O), and (iii) large amounts of ¹⁸O in the subsurface of the ceria support due to oxygen transfer from the surface to the ceria subsurface, highlighting the oxygen dynamics of the ceria support. While the results of this study are fully consistent with a redox mechanism involving a reaction pathway for replenishment of surface oxygen ions O²⁻ from terminal hydroxyl groups (O–H) accessible also in the absence of CO in the gas phase, other reaction mechanisms cannot be ruled out.