The Fischer-Tropsch synthesis (FTS) reaction mechanism has been studied by establishing a steady-state operation using a syngas comprised of H-2 and CO and then switching to a syngas feed comprised of D-2 and carbon monoxide. The products made during the 24-h period following switching from a hydrogen-containing syngas to a deuterium-containing one are analyzed for deuterium content. The paraffin/olefin (P/O) ratio of the highly deuterated products represent primary FTS products. The data show that the P/O ratio increases much more slowly with carbon number than the O/P ratio of the total products exiting the reactor. The data show that diffusion limitations for the olefin products and their subsequent re-incorporation as chain initiators does not make a major impact on the product distribution. (c) 2005 Elsevier B.V. All rights reserved.