The phrases "product accumulation" or "accumulated products" or "product holdup" have appeared in literature during the past several decades to qualitatively explain the experimental results for Fischer-Tropsch synthesis (FTS). This study develops an experimental method for a slurry reactor to evaluate the product accumulation inside the FT reactor by measuring the average residence time of products as a function of carbon number. The effect of accumulation of products on vapor-liquid equilibrium (VLE) in the reactor is also investigated. The results show that VLE is reached inside the FT reactor for components up to C-17. Furthermore, the relationship between the mole fractions of components in the vapor and the liquid phase for lighter hydrocarbons, up to around C-17, is adequately described by Raoult's law. These results suggests that chain length-dependent solubility in the liquid phase is the predominant cause for chain length dependencies of secondary olefin reactions in FTS, and diffusion-limited removal of products is only significant for products with carbon number greater than 17. (C) 2011 Elsevier Inc. All rights reserved.