The mass transfer of the system CO2-water was measured up to a pressure of 360 bar at a temperature of 25 degreesC and the results were compared to empirical predictions. The influence of a surfactant on the mass transfer was investigated. A Schlieren method and a three-mode magnetic suspension balance connected to an optical cell were used to analyse the mass transfer. A good simulation of the experimental values was achieved by linear superposition of free convection and Marangoni convection. The surfactant enforces the Marangoni convection while the solubility of CO2 stayed unaffected. (C) 2004 Elsevier Ltd. All rights reserved.