Grand canonical Monte Carlo simulations were used to explore the adsorption behavior of methane, ethane, ethylene and carbon dioxide in isoreticular metal-organic frameworks, IRMOF-1, non-interpenetrated IRMOF-8 and interpenetrated IRMOF-8. The simulated isotherms are compared with experimentally measured isotherms when available and a good agreement is observed. In the case of IRMOF-8, the agreement is much better for the interpenetrated than for the non-interpenetrated model, suggesting that the experimental data was obtained on an essentially interpenetrated structure. Simulations show that carbon dioxide is preferentially adsorbed over methane and show also a selective adsorption at low pressures of ethane over ethylene, especially in the case of IRMOF-8, confirming very recent experimental results. Analysis of simulation results on both the interpenetrated and the non-interpenetrated structures shows that interpenetration is responsible for the higher adsorbed amounts of ethane at low pressures (< 100 kPa) and for the interesting selectivity for ethane in ethane/ethylene binary mixtures. Van der Waals interactions seem to be enhanced in the interpenetrated structure, favoring ethane adsorption. This indicates that interpenetrated MOF structures may be of interest for separation of small gas molecules.