We report a systematic theoretical study of the influence of pore width, intermolecular potential parameters, and state condition on the selective adsorption of trade components (e.g., pollutants or odorants) from simple fluid mixtures. The pores are of slit shape, and the carrier fluid is taken to be methane. The range of fluid potential parameters chosen embraces common constituents of natural gas. Calculations are based on the nonlocal density functional theory of Kierlik and Rosinberg and show the influence of the relevant variables on the selectivity for the trace component at infinite dilution in the bulk fluid phase. Fluid pressures and pore sizes that lead to a maximum in the selectivity are found, suggesting optimal designs for adsorbents and adsorption processes for trace removal. At these optimum values, extremely large selectivities are possible, particularly at low temperatures.