The adsorption behavior of benzene/propylene mixtures in ZSM-5 zeolite at supercritical conditions has been studied by using Monte Carlo simulation. To guarantee that our simulation is performed at supercritical conditions, we used the Peng-Robinson equation to calculate phase diagrams of benzene-propylene mixtures at different feed ratios of benzene/propylene. Based on the experimental data at supercritical conditions where the optimal reaction ratio of benzene/propylene is 1: 1, we predicted that the optimal feed ratio (R-f) of benzene/propylene is 5: 1, in which the equilibrium ratio of benzene/propylene in ZSM-5 is 1:1. At the optimized feed ratio of R-f = 5.0, we also calculated the adsorption behavior of benzene/propylene in ZSM-5 at other conditions, such as gas, vapor and liquid phase conditions. We observed that the near supercritical condition is more suitable for the benzene alkylation with propylene in ZSM-5 than other conditions. Therefore, we recommend that the feed ratio of benzene/propylene of 5:1 at near supercritical condition is an optimal operative condition for synthesis of cumene via the benzene alkylation with propylene in ZSM-5. (c) 2006 Elsevier B.V. All rights reserved.