The reactivity of Fe+ toward OCS on both quartet and sextet potential energy surfaces have been investigated at the B3LYP/6-311+G(d) level of theory. Various intermediates, transition states, and products involved in the reaction of Fe+ with OCS are presented, and the details of the mechanisms of C-S and C-O bond activations are revealed. The calculated results indicate both the C-O and C-S bond activations proceed according to an insertion-elimination mechanism, which could be general for the reactions of first-row transitions with small sulfide molecules. The C-S bond activation is found to be much more favorable than the C-O bond activation. The energetically most favorable reaction is Fe+(D-6) + OCS --> FeS+((6)Sigma(g)) + CO, endothermic by 1.8 kcal mol(-1). The ground states of products FeCO+ and FeCS+, which cannot be specified by early experiments, have been confirmed to be in their quartets. All theoretical results are in good agreement with early experimental observations.