We have revisited the water gas shift reaction catalyzed by iron pentacarbonyl at the DFT-B3LYP level. The reaction mechanism proposed by Rozanska and Vuilleumier (Inorg. Chem. 2008, 47, 8635-8640) has been followed and revised. The results show that transition states TS4/5 and TS5/2_a actually connect other intermediates rather than those suggested by Rozanska and Vuilleumier. Furthermore, the entire reaction has been proven to proceed with processes 1 -> 2 -> 3 -> 4 -> 6 -> 7 -> 2. It is the first time that species 6 and 7 are reported as intermediates for this reaction mechanism.