The possible application of membranes for CO2 separation in the treatment of non-valuable streams (e.g., flue gas of a power plant or cement industry) or valuable streams (e.g.,biogas) has been analyzed. Some selection criteria useful in the choice of membrane gas separation for CO2 capture are discussed to evaluate the advantages potentially offered by membrane systems. Membrane selectivity ranging from 30-50 (values of commercial membranes) to 100-500 (values of most promising laboratory membranes) and different feed/permeate pressure ratios were considered for the various cases. The composition and recovery of carbon dioxide in the membrane-treated stream were the target parameters taken into account as guidelines in the evaluation of the separation technology performance. General "maps" of CO2 permeate concentration versus CO2 recovery have been developed by means of a simple tool that takes into account the influence of the most importantp arameters affecting the membrane system performance (that is,membrane selectivity and permeation driving force).The analyses indicated that the separation depends on various interrelated factors: the membrane material (selectivity and flux), the operating conditions (pressure ratio), and the final requirements (CO2 recovery and composition).Also, the operational limit and the potentialities of the membrane gas separation technology were analyzed under these conditions.The "maps" proposed and utilized for CO2 separation are valid and can beutilized for other gas separations in which the membrane shows selectivities similar to those taken into account here.