K-value formulations are developed for use in performing vapor-liquid equilibria (VLE) calculations on Fischer-Tropsch (F-T) systems. Carbon-number-based pseudocomponents are defined with the aid of the Anderson-Schulz-Flory (ASF) molecular weight distribution. Proper selection of the number and types of pseudocomponents enables the trade-off which exists between accuracy and computing time to be adequately addressed. The properties of a pseudocomponent are based on a hypothetical 'model' component in each carbon-number cut. Solubilities of gases and light hydrocarbons in the liquid phase are calculated from correlations for the individual solutes in pure n-paraffin solvents, using average carbon numbers of the liquid phase. Comparisons are made with data for several different F-T reactor compositions and with predictions from literature models. The VLE model developed accurately predicts product phase splits, and is superior to other formulations.