The mechanism of fuel NO formation from pyrrole type nitrogen and the effects of H(2)O concentration on the reaction mechanism were examined experimentally using a flow reactor and by numerical simulation with a detailed kinetic model. It was found that when H(2)O concentration is increased, the O radical is consumed and the OH radical is produced by the reaction O + H(2)O -> OH + OH. Consequently, the reaction HCN + OH, the reaction by which HCN is consumed, is promoted; and the reaction NCO + O -> NO + CO, the reaction by which NO is produced, is inhibited.