The adsorption of benzene (B), toluene (T), and p-xylene (X) at ambient temperature in carbon slit pores was studied using the grand canonical Monte Carlo simulation. We focused on how the structure of the adsorbed B, T, and X molecules is influenced by pore size, in particular the effects of the methyl groups in the cases of toluene and p-xylene. The packing density of the compounds in the pores was found to follow the order B < T < X at low loadings but X < T < B at high loadings. Furthermore, various molecular orientations were observed in the different pore sizes, including parallel, perpendicular, and parallel combined with perpendicular, and the nearest neighboring molecules assumed to undergo an edge-wise or T-shaped interaction. For parallel molecules, two adjacent molecules were aligned in an edge-wise interaction, which is the most favorable for solid-fluid interactions. However, for perpendicular molecules, two adjacent molecules were arranged in a T-shape, which is more favorable for fluid-fluid interactions. For the perpendicular toluene and p-xylene molecules, the angle between the methyl group and the pore walls was found to be 30 degrees. In the smallest pore studied, the benzene molecules were found to adopt a 2D hexagonally packed solid-like state, whereas the toluene and p-xylene molecules were in a liquid-like state. Consequently, the microscopic structures of B, T, X in various sized pores were investigated. The molecular models showed that the fluid-fluid interactions were significant for B, T, X adsorption, except for the cases of toluene and p-xylene in pores that can accommodate more than one molecular layer, where the solid-fluid interactions play a more dominant role because of the enhanced solid-fluid interactions of the toluene and p-xylene molecules oriented in parallel to each other. In addition, we have compared the results of our simulation with experimental data from previous research. The simulated isotherm agrees reasonably well with the experimentally obtained isotherm, and the simulated nearest-neighbor distance between the adsorbed molecules is almost the same as the experimental value. (C) 2018 Elsevier Ltd. All rights reserved.