We analyzed the mechanisms of the water-gas-shift reaction catalyzed by Fe(CO)(5)/OH- in the gas phase using DFT methods. The systematic analysis of the accessible reaction mechanisms and the consideration of the Gibbs free energies allows for different reaction routes than previously suggested. In the dominant catalytic cycle, the hydride [FeH(CO)(4)]- is the important intermediate. Associative reaction mechanisms are not favorable under moderate and low pressures. At high pressure, a side reaction takes over and prevents the conversion of H2O and CO to H-2 and CO2 and leads to the formation of HCOOH.