The adsorption of pure ethane and carbon dioxide, and binary mixtures of these components, in MCM-41 has been studied experimentally and by grand canonical Monte Carlo (GCMC) simulation, at temperatures between 264 and 303 K and pressures up to 3 MPa. The experimental isotherms were measured using a bench-scale, open-flow adsorption/desorption apparatus. The simulations were carried out using three different models for MCM-41 with different degrees of surface heterogeneity. Model 1 has a nearly homogeneous pore surface while model 2, which is derived from a matrix of crystalline alpha-quartz, has a heterogeneous but nevertheless regular surface. These two models give generally good predictions of the adsorption of ethane in MCM-41, except at low pressures where the surface heterogeneity of MCM-41 dominates the adsorption. Model 3 has an amorphous structure, generated by an energy-minimization procedure; this model gives better predictions for ethane adsorption, especially at low pressures, suggesting that it incorporates a good representation of the heterogeneity of the real MCM-41 material. Excellent predictions of the adsorption of pure carbon dioxide and binary mixtures of ethane and carbon dioxide in MCM-41 are obtained with model 3, further confirming the realism of this model. Long-ranged electrostatic interactions are included for the simulation of carbon dioxide; these interactions, which play an important role, are treated by a simple one-dimensional summation method, which gives an accurate calculation of the potential.