We demonstrate Chemical Transient Kinetics as a powerful tool for investigating catalytic reaction kinetics and mechanisms at atmospheric pressure. Quantitation of transient molecular flows while switching feed gas compositions allows key mechanistic steps of the Fischer-Tropsch reaction to be elucidated. Evidence is presented for a CO insertion mechanism operating under hydrogen-deficient reaction conditions. Consequently, formate/carboxylate-derived intermediates are likely involved in chain lengthening. This also agrees with the occurrence of characteristic time profiles when replacing CO by methyl formate or adding ammonia to a CO/H(2)feed. Graphic