We have used in situ X-ray photoelectron spectroscopy to obtain information about the chemical state of a Fe single-crystal catalyst upon addition of CO₂ in the syngas feed during Fischer–Tropsch synthesis (FTS) between 85 and 550 mbar. We found that at certain temperatures, the ternary mixture of CO, CO₂, and H₂ yields a chemical state which is resemblant of neither the CO hydrogenation nor the CO₂ hydrogenation reaction mixtures in isolation. The addition of CO₂ to a CO + H₂ reaction mixture mostly affects the chemical state at low-temperature FTS conditions (i.e., below 254 °C). In this temperature span, the ternary reaction mixture resulted in a carburized surface, whereas the CO + H₂ reaction led to surface oxidation. We propose a hypothesis, where a carbonate intermediate produced by CO₂ interaction with Fe oxide aids the reduction of the Fe oxide, paving the way for the carburization of the Fe by dissociated CO. Very small differences in the spectra of the CO + H₂ and the CO + CO₂ + H₂ reaction mixtures were observed above 254 °C, suggesting that the CO₂ is a spectator in these conditions. Changing the total pressure of both the CO hydrogenation and ternary reaction mixture causes quantitative changes in the spectra at both low- and high-temperature FTS conditions, the degree of oxidation/carburization was affected in the low-temperature-FTS regime, and the degree of hydrocarbon build-up was affected in the high-temperature-FTS.