Polymers of intrinsic microporosity (PIMs) have emerged as a special class of polymeric materials and received considerable interest for many potential applications. Nevertheless, most current studies for PIMs are largely focused on gas separation. In this study, we report a simulation protocol to construct a thin membrane of PIM-1, examine its swelling in water and ethanol/water mixture, and explore its performance for water desalination via reverse osmosis as well as biofuel dehydration via pervaporation. The predicted density of the PIM-1 membrane matches well with experimental data. The estimated gravimetric and volumetric swelling degrees are in accord with measured values. As ethanol has a greater capability than water to swell the membrane, the void distribution of the membrane is found to shift to a larger size in ethanol/water mixture than in water. For water desalination, the membrane exhibits water permeability of 8.6 x 10(-7) kg.m/(m(2).h.bar) and 100% salt rejection, which is higher than commercial reverse osmosis membranes. For the dehydration of ethanol/water mixture, water permeability and separation factor are 5 x 10(-5) kg.m/(m(2).h.bar) and 25, respectively, surpassing poly (vinyl alcohol) and polybenzimidazole membranes. This simulation study provides microscopic insights into the structural and separation properties of PIM-1 membrane from the bottom-up, and suggests that PIM-1 membrane might be an interesting candidate for water desalination and biofuel dehydration.