Metal-organic frameworks (MOFs) have emerged as a new class of adsorbents with the potential for making an important impact in adsorption applications. MOFs are synthesized by a self-assembly process in which organic linkers coordinate with metal centers or clusters. Cu-BTC is a prototypical MOF constructed from copper and benzene tri-carboxylic acid. This material possesses a zeolitic interconnected network of pores in which copper atoms are built into the pore walls in a regular fashion. These metal sites are also coordinatively unsaturated, and thus, available for interaction with guest molecules. Traditional adsorbents such as zeolites and impregnated carbons are well known to possess selective adsorption and catalytic properties that can be attributed to the availability of open metal sites within the pores. It follows then that designing MOFs with open metal sites is a viable strategy for tailoring the adsorptive selectivity of these materials. In this work, we explore the role of open metal sites in the adsorption of carbon dioxide, carbon monoxide, and methane in Cu-BTC. Cu-BTC was synthesized and characterized in our lab. Adsorption equilibrium data were measured at 298 K. Grand canonical Monte Carlo simulations were also performed in conjunction with experiments for examining preferred adsorption sites and binding energies for the three molecules. In this paper, we will present these results and will discuss the role of the open metal sites in the framework compared to surface area/void volume considerations in affecting the adsorption behavior of pure gases and mixtures.