Significant reductions in interstitial iron (Fei) concentrations occur during annealing Fe-containing silicon wafers with silicon nitride films in the temperature range of 250 °C–700 °C. The silicon nitride films are known to release hydrogen during the annealing step. However, in co-annealed samples with silicon oxide films, which are hydrogen-lean, changes in the Fei concentrations were much less significant. The precipitation of Fei is ruled out as a possible explanation for the significant reductions. The hydrogen passivation of Fei, which is the complexing of monatomic H and isolated Fei forming a recombination-inactive hydride, is proposed as the most probable model to explain the reductions. Under the assumption that the reduction is caused by the hydrogenation of Fei, the reactants' charge states in the hydrogenation reaction are determined by two independent approaches. In the first approach, illumination is found to have a small but detectible impact on the reaction kinetics in the lower temperature range. The dominating reactants' charge states are concluded to be Fe0 + H+ as revealed by modelling the injection-dependent charge states of isolated Fei and monatomic H. In the second approach, the reaction kinetics are fitted with the Arrhenius equation over a large temperature range of 250 °C–700 °C. A reasonable fit is only obtained when assuming the reacting charge states are Fe0 + H+. This supports the conclusion on the reacting charge states and also gives a value of the activation energy of hydrogenation in the 0.7–0.8 eV range.