We report calculations, based on density functional theory and on molecular simulations, for a simple model of the selective adsorption in slit carbon pores for three trace contaminants, carbon tetrachloride, carbon tetrafluoride, and sulfur dioxide, from nitrogen streams at 300 K. The influence of gas pressure, pore width, and concentration has been studied. In each case there is an optimal pore width for which the selectivity is greatly enhanced. Also, there is usually an optimal pressure that gives a maximum selectivity. This effect is less pronounced than that for pore width; the maximum occurs when the adsorbed monolayer is nearly complete. We show that, except at higher pressures, the nitrogen carrier molecules do not compete effectively with the trace component for adsorption. The bulk mole fraction that yields the infinite dilution limit in the pore is found to be simply related to the selectivity. The results obtained are for materials with monodisperse pores. To estimate the effect of pore size distribution (PSD) on selectivity, we make a comparison with a microporous carbon, AC610, for which the PSD is known, The PSD is found to significantly degrade the selectivity, as expected. The density functional theory is in very good agreement with the simulation results for these infinitely dilute mixtures.