The adsorption of hydrocarbon (pure CH4 and C2H6) on illitic clay was investigated at temperatures of 333, 363, and 393 K (60, 90, and 120 degrees C) over a range of pressures up to 30 MPa using grand canonical Monte Carlo (GCMC) simulations. We first discussed the comparability of molecular simulation results with experimental measurements. Our results indicate that molecular simulation results of the excess adsorption are comparable with the experimental measurements if they are both expressed per unit surface area available for adsorption instead of per unit mass. The gas density profiles indicate that the adsorption of CH4 and C2H6 is mainly affected by the clay surface layers. In micropores smaller than 2 nm, the overlapping of the interaction of the simulated pore walls with the gas results in enhanced density peaks. For pore sizes of 2 nm or larger, the overlapping effect is significantly reduced, and the height of the gas density peak close to the surfaces is no longer affected by pore sizes. The maximum excess adsorption of illite for C2H6 is almost twice that for CH4 due to the stronger interaction between illite and C2H6 than between illite and CH,, but the saturation capacity (maximum loading) is the same for both. Our findings may provide some insights into gas adsorption behavior in illite-bearing shales and give some guidance for improving experimental prediction.