Gas-liquid mass transfer in bubble columns is strongly influenced by the flow field in the column. The underlying transport processes are often the limiting factor. Since bubble columns are widely used in the chemical process industry, it is essential to quantify how the dispersed gas can come into reaction with the liquid phase through gas-liquid mass transfer. CO2 bubble swarms in a cylindrical model bubble column are considered. Optical measurement methods are used for the non-intrusive measurement of velocity and pH change, resulting from the dissolution of CO2 in the liquid phase. The mean and fluctuating velocity components in the center plane of the column as well as the time-dependent pH fields in this plane are determined. From these quantities, derived values characterizing the mass transfer from gaseous CO2 to the liquid phase can be calculated.