Conventional Fischer-Tropsch (FT) experiments were performed in both a continuously stirred tank reactor (CSTR) and a plug-flow reactor (PFR) on a TiO2-supported cobalt catalyst. The olefin/paraffin ratios for short-chain hydrocarbons (C-2-C-5) were found to change when the reaction conditions were kept constant in a CSTR, while the ratio of adjacent olefins (for example, C4H8/C3H6) remained unchanged. In the experiments with a fixed-bed reactor, the temperature was varied from 210-250 degrees C and different flow rates ranging from 1.8-5.4 NL h(-1) g-cat(-1). The ratio of adjacent olefins was found to be constant under these conditions as well. After proposing a thermodynamic model for olefin product distribution, we compared the results that the model predicted with those obtained in the experiments, and found that there is good agreement between the thermodynamic predictions and the measured distributions. This supports the postulation that the olefin distribution in the Fischer-Tropsch synthesis (FTS) reaction might be determined by thermodynamics.