A critical step in developing adsorption-based technologies is the identification/synthesis of adsorbents that provide the proper adsorption capacities, selectivities, or reactivities for the application. It is well known that metal sites in porous materials have a tremendous influence on the resulting adsorption properties. Metal-organic frameworks (MOFs) are a recent addition to the classes of porous materials and have the potential for providing a flexible platform for developing designer adsorbents. MOFs are synthesized by self assembly of organic ligands and metal oxide clusters and possess pore sizes and chemical functionalities that can be manipulated by modifying the metal group or organic linker. Several MOFs have open metal sites (coordinatively unsaturated) that are built into the pore walls in a repeating, regular fashion. These metal sites have been shown to impart catalytic activity to the materials and also have the potential to enhance general adsorption properties. In this work, we explore the effect of open metal sites in Cu-BTC on adsorption selectivities of carbon monoxide- and carbon dioxide-containing mixtures. Molecular modeling results and experimental results will be discussed.