Under a pressure tuned supercritical fluid mixture of pentane/hexane as the media and using Co/Al2O3 as the catalyst, the Fischer-Tropsch synthesis (FTS) reaction was conducted in a fixed bed reactor. C-14-labeled decene, tetradecene, and nonadecene were chosen as probe compounds to study the chemical behavior of the added mid-carbon number olefin under FTS supercritical reaction conditions. In the case of the run with decene, 21.8% of the added decene was reduced to decane, 73.5% remained unchanged, and only 4.7% of the added decene was incorporated into higher carbon number compounds. For the tetradecene and nonadecene runs, the percentages of reduction of the added olefin are 23.4 and 2.5%, while the percentages of incorporation are 2.6 and 1.5%, respectively. The molar radioactivity of the incorporated compounds decreases with increasing molecular size, indicating the presence of some accumulated products under these reaction conditions. The a value calculated based on the relative radioactivity of the incorporated compounds is 0.62, which is smaller than the value obtained from normal FTS under the same reaction conditions. These results indicate that the added mid-carbon number olefin cannot significantly alter the product distribution in favoring the heavier products. (c) 2005 Elsevier Ltd. All rights reserved.