The densities of the carbon dioxide (CO2) and hydrogen (H-2) mixtures (x(H2) = 0.020, 0.075 and 0.100) have been determined at six temperatures from 288.15 to 333.15 K and pressures up to 23.0 MPa using a high-pressure vibrating-tube densitometer. The experimental temperatures and pressures cover a range of sub- and supercritical conditions, providing essential information for the optimum design and operation of compressors and pipeline networks in carbon capture and storage (CCS). It was found that a concentration of H-2 as low as 2% could lower the density by as much as 25% compared to pure CO2. The data were used to calculate the excess molar volumes, showing highly non-ideal mixing behaviour of the binary system of CO2 + H-2. The new density data were also compared to those predicted using the GERG-2004 equation of state (Kunz, O. et al., 2007. The GERG-2004. Wide-range equation of state for natural gases and other mixtures, Dusseldorf). The deviations between the calculated and experimental data are 0.6%, 1.7% and 1.8%, respectively, for the mixtures with x(H2) = 0.020, 0.075 and 0.100. These results suggest that the GERG-2004 equation of state has the potential for accurate prediction of the volumetric property of CO2 mixtures containing H-2 in the CCS processes, and further parameterisation of GERG-2004 or development of new equations of state specifically tailored for CCS mixtures is required. (C) 2012 Elsevier Ltd. All rights reserved.