The addition of D-5-1-pentene[4,4,5,5,5] to the synthesis gas fed to cobalt and iron catalysts under Fischer-Tropsch synthesis conditions show that H/D exchange does not occur to a measurable extent. The added D-5-1-pentene[4,5,5,5] underwent hydrogenation, isomerization to interval alkenes and initiated chain growth to a greater extent with the cobalt than the iron catalyst. The data show similar results for both catalysts with the readsorbed alkene initiating chain growth, but not in an amount to significantly impact the product distribution. For cobalt, but not for iron, the added alkene may also be involved in the hydroformylation reaction. The data clearly show that vapor pressure and product accumulation effects must be accounted for in mechanism studies using isotopic tracers. The added D-5-1-pentene[4,4,5,5,5] was found to initiate a growing hydrocarbon chain as the CD3CD2CH2CH2CH2 unit without measurable scrambling of the isotope label in contrast to earlier work with deuterated ethanol using a cobalt catalysts. (c) 2010 Elsevier B.V. All rights reserved.