Carbon capture and storage (CCS) is one of the major transformative technologies for reducing atmospheric CO2 emissions from large CO2 emitters where a large potential for geological CO2 storage exists. CO2 captured from the emitters contains various impurities, such as N-2, O-2, Ar, SOx, etc., and allowing these impurities to be stored together with CO2 would reduce the cost of CO2 capture. However, the impurities would have various undesirable effects on CO2 storage. In this work we analyze the impact of non-condensable impurities N-2, O-2 and Ar, which are present in large quantities in oxyfuel CO2 streams, on CO2 storage capacity of geological formations. A formula for measuring the impact of the impurities on CO2 storage capacity is proposed, and based on the formula important insights are obtained. It has been shown that the impurities in oxyfuel flue gas reduce the structural trapping capacity for CO2 by reducing the density of CO2, and the reduction of the trapping capacity is greater than the volume fraction of the impurities. Particularly, there is a maximum reduction of the storage capacity at a certain pressure, and this pressure changes with temperature. The strategy for alleviating the negative impact of the impurities has been discussed. Crown Copyright (c) 2012 Published by Elsevier Ltd. All rights reserved.