Laboratory measurements of the diffusion coefficient of CO2 into brine under both unsteady state and steady state conditions were carried out to obtain the dissolution rates of CO2 during CO2 injection and/or flooding in short term and for CO2 preservation at thermodynamic equilibrium in long term. The experiments used pure CO2 gas and a synthetic brine of 2% NaCl with experimental P/T conditions at 2580 Psia and 83 °C, respectively, similar to that of the target reservoir interval of the proposed CO2 injection well (CRC-1) in the Otway Bain, Australia. The pressure drop in the core flooding cell during the contact of the CO2 and brine phases was used to measure the diffusion coefficient of the system under the storage condition. The diffusion mechanism includes both a steady state and an unsteady state component. The diffusion coefficient of the CO2 in the unsteady state appears to be larger than that of the steady state. The duration of the unsteady state appears to strongly depend on the pressure and temperature of the system.