Depleted hydrocarbon reservoirs and deep saline aquifers are key targets for geological storage of CO2 to reduce atmospheric CO2 emissions. Most studies in CCS investigate subsurface storage of pure CO2. In this paper we investigate the impact of the presence of other gases (impurities) in the injected CO2 stream on solubility trapping (in the aqueous phase) and volumetric trapping (in the non-aqueous phase, for a wide range of pressure and temperature. Calculations for solubility trapping are based on an equation of state that accurately accounts for the pressure, temperature, gas-compositional (mixtures) and salinity influences on CO2 solubility and brine density. For volumetric trapping the Peng-Robinson equation of state is used, accounting for binary interaction for gas mixtures and density correction. In the analysis, special attention is paid to the impact of SO2, which exhibits anomalous storage effects when compared to other common impurities. It is shown that while most impurities reduce the CO2 storage capacity (STC) in both the aqueous and non-aqueous phase, presence of SO2 enhances STC in both phases for a wide range of pressure and temperature conditions. However, for the realistic amounts of SO2 in flue gases, the effects are rather small; for a SO2 content of about 0.5% the non-aqueous STC enhancement ranges up to about 4%. For volumetric trapping, the greatest positive impact of SO2 occurs at relatively low pressures (74-100 bar) and temperatures (313-325 K). These are typical for shallow (<1 km) aquifers or deeper depleted hydrocarbon reservoirs during the injection stage. For solubility trapping, the STC enhancement by SO2 increases with pressure and is relatively insensitive to temperature, implying that the greatest positive effect would be achieved for deep saline aquifers. These findings suggest that the positive effects of SO2 on the CO2 storage capacity could be of practical significance for CCS projects. The positive storage effects would have to be evaluated relative to possible negative effects due to induced geochemical reactions, corrosion of well casings, and health risks associated with potential leakage from transport or injection facilities or from the storage reservoir. (C) 2014 Elsevier Ltd. All rights reserved.