The addition of a hydrocarbon, n-hexane, to the feed of the iron-catalyzed Fischer-Tropsch synthesis in a fixed bed reactor operating at a total pressure of 60 or 90 bar is shown to be beneficial, i.e. resulting in a higher activity with only a slight increase in methane selectivity. The thermodynamic properties of pseudo-binary systems containing H-2:CO:n-hexane (2:1:y)-long chain hydrocarbon were investigated using the Peng-Robinson equation of state. The chain length of the long chain hydrocarbon is systematically varied to investigate the thermodynamic behaviour of the system in real waxes. It is concluded that supercritical conditions require pressures in excess of those applied in this study (and those reported in the literature). Thus, a supercritical phase is not formed. It is further shown that a three-phase system is likely to be present in the reactor under the typically applied reaction condition for the so-called 'supercritical' Fischer-Tropsch synthesis. The reported beneficial effect of the added hydrocarbon to the feed of the Fischer-Tropsch synthesis might be attributed to the change in the partial pressure of the reactants, hydrogen and carbon monoxide. The addition of a hydrocarbon can further aid in obtaining primary products of the Fischer-Tropsch synthesis due to increase in the liquid flow rate through the reactor. (C) 2010 Elsevier B.V. All rights reserved.