The promotional effect of sodium on Pt/Al2O3 catalysts for the water-gas shift (WGS) reaction was investigated with operando FTIR during steady-state and isotopic transient experiments. The highest turnover frequency (TOF) promotion per surface Pt occurred on a 0.82 wt.% Pt/Al2O3 catalyst with a 30:1 molar ratio of Na:Pt. This catalyst exhibited a TOF of 0.35 s(-1) at 250 degrees C and 7% CO, 11% H2O, 9% CO2, and 37% H-2 compared to 0.43 x 10(-2) s(-1) on Pt/Al2O3. Operando IR revealed that Na addition modifies CO adsorption on Pt to create more strongly bound, multiply-bonded CO species at 1768 cm(-1) and 1697 cm(-1) compared to predominantly linear CO on Na-free Pt/Al2O3 at 2060 cm(-1). Transient experiments with (CO)-C-12/(CO)-C-13 isotope switches showed that the number of carbon-containing active intermediates increased from less than 1% of the surface Pt for Na-free Pt/Al2O3 to nearly 100% of the surface Pt for 20Na:Pt/Al2O3, which indicates that only a small fraction of the Pt surface on the Na-free samples participates in the WGS reaction. From time-resolved IR spectra during transient WGS with (CO)-C-12/(CO)-C-13 and (CO2)-C-12/(CO2)-C-13 isotope switches, we propose that surface formate species are spectators for all Na-promoted Pt/Al2O3 catalysts under these WGS conditions. The results suggest that Na promotes Pt/Al2O3 for WGS by modifying the local electronic properties of Pt and creating new H2O activation sites, which provide greater availability of surface OH/H species to react with nearly all CO on the metallic Pt surface through a non-formate pathway. (C) 2016 Elsevier Inc. All rights reserved.