The catalytic hydrogenation of carbon monoxide, known as the Fischer-Tropsch process, is a technologically important, complex multipath reaction which produces long chain hydrocarbons. In order to access the initial kinetics and the mechanism, we developed a reactor that provides information under non-steady state conditions. We tested a CoMgO catalyst and monitored the initial product formation within 2 s of exposure to CO as well as the time dependence of high molecular weight products (in a 60 s window) and found drastic changes in the product selectivity. The probability for forming branched isomers (C-4 and C-5) peaks in the first 25 s, and within that time frame no unsaturated products were detected. The subsequent decline (at 35 to 40 s) of branched isomers coincides with the detection of olefins (from C-2 to C-5) and the change in carbon coverage at the surface of the catalyst. This indicates a change in the reaction pathway. (C) 2016 Elsevier B.V. All rights reserved.