This investigation measured the impact of the liquid starting medium on syngas (H-2+CO) conversion via the Fischer-Tropsch (F-T) synthesis in a 11 continuous stirred tank reactor (CSTR). The experiments were conducted with a low-wax producing iron catalyst, so that the liquid medium composition was unaltered by accumulation of product wax throughout the experiment. Our results show a greater deactivation rate for the F-T reaction in heavier starting media (Polywax-655, Allied AC-1702, and Mobil F-T product wax) versus the lighter medium (n-octacosane). The syngas (H-2+CO) conversions were approximately equal and steady for the first five days in both light and heavy, commercially available wax media. However, the conversions dropped more rapidly in the heavier wax media (from 85% to 60% in 12-13 days), but only modestly in the light wax media (from 90% to 85% in 15 days). An even sharper deactivation rate occurred with the heavy Mobil F-T wax (from 85% to 60% in six days). For all experiments, the water gas shift (WGS) ratio in the outlet gases dropped concurrently. One proposed explanation for this increased deactivation in the heavier waxes is that heavier products are being formed by secondary reactions. These products could build up over time and restrict the diffusion of hydrocarbon products and water vapor out of the pores. The presence of higher partial pressures of water vapor could irreversibly oxidize the catalyst.