The separation of xylene isomers is one of the most difficult separations in petrochemistry. Adsorbents have to preferentially adsorb para or meta isomers, but there are only a few zeolites which fulfill the criteria of selectivity and adsorption capacity. In this study, we evaluate two metal?organic frameworks (MOF) with coordinatively unsaturated metal sites (cus), i.e., CPO-27-Ni and HKUST-1, in the adsorption and separation of p-xylene, m-xylene, and o-xylene in the gas phase. The results are compared with those of the zeolite NaY. CPO-27-Ni and HKUST-1 are both ortho-selective adsorbents, but molecular simulations indicate that the reasons for the ortho selectivity are quite different. The CPO-27-Ni structure is intrinsically ortho selective, and the ortho selectivity is further enhanced by electrostatic effects. HKUST-1, on the other hand, is intrinsically (weakly) para selective but the para selectivity is overcompensated by electrostatic effects, which overall leads to a slight preference for the ortho isomer adsorption. The insights in the adsorption behavior of xylenes that are provided by this study provide useful guidelines in the quest for para-selective MOFs materials. The separation of xylene isomers is one of the most difficult separations in petrochemistry. Adsorbents have to preferentially adsorb para or meta isomers, but there are only a few zeolites which fulfill the criteria of selectivity and adsorption capacity. In this study, we evaluate two metal?organic frameworks (MOF) with coordinatively unsaturated metal sites (cus), i.e., CPO-27-Ni and HKUST-1, in the adsorption and separation of p-xylene, m-xylene, and o-xylene in the gas phase. The results are compared with those of the zeolite NaY. CPO-27-Ni and HKUST-1 are both ortho-selective adsorbents, but molecular simulations indicate that the reasons for the ortho selectivity are quite different. The CPO-27-Ni structure is intrinsically ortho selective, and the ortho selectivity is further enhanced by electrostatic effects. HKUST-1, on the other hand, is intrinsically (weakly) para selective but the para selectivity is overcompensated by electrostatic effects, which overall leads to a slight preference for the ortho isomer adsorption. The insights in the adsorption behavior of xylenes that are provided by this study provide useful guidelines in the quest for para-selective MOFs materials.