The density driven convection phenomenon is expected to have a significant and positive role in CO2 geological storage capacity and safety. But predictions on reservoir time and space scales are difficult to validate because data are generally sparse and will only be useful for a small part of the relevant time period. Laboratory scale data are valuable to validate the numerical models. In this paper we focus on the comparison of experimental and numerical determination of CO2 mass transfer in a laboratory experiment. We developed an experimental protocol for the determination of density-driven mass transfer of CO2 in water-saturated Hele-Shaw cells with different apertures. We used a CCD camera to capture images of the initiation of density-driven convection caused by dissolution of CO2 in water and the subsequent development of convective fingers. The visualization of the phenomenon allowed consistently stopping the experiment when dissolved CO2 first reached the bottom of the cell. We determined the total mass of dissolved CO2 during the experiment using a catalytic combustion-based total carbon analyzer (TC-analyzer). This experimental procedure was repeated several times for uncertainty analysis. Thus a combination of quantitative and qualitative experimental results for the same Hele-Shaw cell configuration was obtained for validation of corresponding numerical simulation results. A numerical simulation of the phenomenon was carried out using the STOMP-WCS simulator. We found that in order to accurately simulate numerically the phenomenon occurring in the Hele-Shaw cell, existent variations in the cell apertures should be taken into account. Thus we observed a good agreement between the experimental and numerical results in terms of total dissolved CO2 mass, timescale of mass transport and morphology of the convection fingers. In addition correlations are obtained between total dissolved CO2 mass, arrival time of dissolved CO2 to bottom of the cell, and the Rayleigh number. (C) 2014 Elsevier Ltd. All rights reserved.