Most adsorbent materials used for olefin/paraffin separation show preferential adsorption of the olefin. Recently, the material aluminum methylphosphonate polymorph alpha (AlMePO-a) was found to be able to selectively adsorb the paraffin instead of the olefin, from an ethyl chloride/vinyl chloride mixture (Herdes, C.; Valente, A.; Lin, Z.; Rocha, J.; Coutinho, J. A. P.; Medina, F.; Vega, L. F. Langmuir 2007, 23, 7299). However, several questions remain still open regarding the reasons for this selective paraffin adsorption, as well as the suitability of AlMePO-a as adsorbent for other olefin/paraffin separations. In this work, the adsorption of ethane/ethylene mixtures by AlMePO-a is investigated using grand canonical Monte Carlo simulations in order to determine the effect of molecular interactions, size, and shape on the selective adsorption. For this purpose three different force fields have been used for the fluids, investigating the effect of the molecular details of the fluid on the adsorption behavior. All three force fields gave the same qualitative behavior. It was found that AlMePO-a is also able to selectively adsorb the paraffin from ethane/ethylene mixtures. Moreover, ethane molecules arrange exactly in the same way in the adsorbent material as ethyl chloride, with the methyl groups directed toward each other, although ethane has much smaller dynamic diameter compared to ethyl chloride. Therefore, a key factor determining the selectivity is found to be the molecular interaction between the methyl group of AlMePO-a and the methyl group of the paraffin.