The ignition of stoichiometric and fuel-lean hydrogen-air mixtures near 10()0 K Was Studied, for varying amounts of iron additives. Depending on the initial conditions, the addition of iron pentacarbonyl promotes or inhibits the ignition of hydrogen/air mixtures. The simulations show that iron compounds at I mu L/L(1) reduce the ignition delay by about 30%, and that 50 mu L/L can reduce the ignition delay by a factor or two or three for lean and stoichiometric conditions: at Slightly higher volume fraction (> 150 mu L/L), the ignition is retarded. The effectiveness of the iron compounds is influenced by the form of the added iron, stoichiometry, and initial temperature of the mixture. At low additive Volume fraction, the promotion is the result of relatively fast decomposition of iron pentacarbonyl. which provides a source of radicals at the initial reaction stage during radical pool build-up the ignition promotion is proportional to the amount of iron-containing additive, and mostly, does not involve cycling reactions. In contrast, at higher additive Volume fraction, the ignition is retarded by the iron compounds. through catalytic radical recombination cycles similar to those acting in flame inhibition. The reduction in chain-carrying radical concentrations occurs at later times in the ignition process when the radicals have reached high concentrations. Published by Elsevier Inc. on behalf of The Combustion Institute.