It is essential to understand the relative permeability of CO2 and brine when characterizing the flow of CO2 as it is injected into an aquifer. The relative permeability of CO2 and brine has a significant impact on CO2 migration and saturation distribution. This paper presents the results of experimental investigations into the effects of the viscosity ratio and interfacial tension (IFT) on steady-state relative permeability. A series of unsteady-state experiments were conducted with a wide range of flow rates. These enabled us to determine the low end-points of the relative permeability and high residual brine saturation of the two-phase CO2/brine system. The results clearly show that the steady-state relative permeability depends on the viscosity ratio and IFr of the CO2/brine system. Due to the low CO2 viscosity and flow rate, the residual saturation of brine is greater than 50%. The low end-point of the relative permeability of CO2 is associated with small viscous forces. In the CO2/brine system, these forces are normally smaller than the capillary force. The small viscous forces are caused by the injected CO2 having a much lower viscosity than brine. The viscous force increases with increasing flow rate, resulting in more efficient CO2 displacement. (C) 2017 Elsevier Ltd. All rights reserved.