CO2 storage in deep saline aquifers has been suggested as a way of reducing greenhouse gas emissions. Correct estimation of dissolution rate is important because the time scale for dissolution corresponds to the time scale over which free phase CO2 has a chance to leak out. However, for this estimation, an expensive and time consuming simulation based on the convection-diffusion equation must be performed. In this paper, a new model for rapid determination of dissolution rate for convection-diffusion mechanism is proposed. It is used to account the rate of mass transferring in porous media during CO2 storage. For this purpose, a new diffusion equation was developed based on the pseudo diffusion coefficient of the system. Experimental tests were conducted in porous media to evaluate the pseudo diffusion coefficient of the system. It is shown that this coefficient can be approximated by a relationship for the Rayleigh number of the system. The fraction of ultimate dissolution, as a result of the proposed model, is calculated for each test and compared with the experimental data to verify the reliability of proposed method.