A new product distribution quasi-steady-state model was proposed for the Fischer-Tropsch (FT) synthesis in slurry reactors, being applicable to their transient simulation. It may consider two chain propagation mechanisms or sites and the possibility of 1-olefin readsorption with secondary reactions, recovering the Anderson-Schulz-Flory (ASF) model, the two superimposed ASF model and the 1-olefin readsorption model as particular cases. The hydrocarbon compounds were lumped according to the number of carbon atoms in their molecules for the paraffin and 1-olefin families. The phases were assumed to be well mixed in the reactor and transient mass balances were performed for each component, allowing simulation of operation in constant liquid level and no liquid withdrawal conditions. A rigorous calculation of the vapor-liquid equilibrium (VLE) through cubic equations of state was used to describe the phase behavior. Rate expressions for the FT and the water gas shift reactions are taken from the literature and expressed in terms of fugacities. Simulation results showed that the inclusion of both the olefin readsorption and the two chain propagation mechanisms may explain the;Anomalies present in experimental hydrocarbon product distribution. Moreover, the effect of phase-equilibrium modeling on product distribution simulation was shown to be slightly important in the conditions analyzed. Compositional lumping schemes for the hydrocarbons were investigated to speed up the simulations. Results showed that lumping can speed up computations up to 250 times with negligible loss of accuracy. (C) 2004 Elsevier Ltd. All rights reserved.