Caldwell and van Vuuren (1986) [1] were the first to realize the importance of vapor-liquid equilibrium (VLE) considerations in the Fischer-Tropsch (FT) synthesis modeling. They used Raoult's law to describe VLE. Recently, a variety of VLE thermodynamic models have been used to model FT products. Thus the aim of this study was to conduct an experiment to measure actual vapor and liquid compositions of long chains hydrocarbons under FT reaction conditions to ascertain whether Raoult's law is sufficient or other elaborate VLE models are required. The results obtained show that VLE is attained inside an FT reactor. The measured vapor and liquid compositions (K-values) can be sufficiently described by Raoult's law. Hydrocarbons with carbon number greater than 18 deviates from Raoult's law. The deviations from Raoult's law are due to diffusion limitations. Elaborate thermodynamic models could be used given the pure component parameters with relevant mixing rules for a higher degree of accuracy. (c) 2011 Elsevier B.V. All rights reserved.