Gas-liquid contacting in mechanically agitated vessels is widely used in the process industry. Bubble size measurements at different now regions in the vessel provide useful information for the mechanisms of gas dispersion and gas-liquid flow. In this work, bubble size distributions and distributions of Sauter mean bubble diameters at four different regions in air-water and air-NaCl solution systems agitated by a six-blade disk turbine are measured by using the photographic method. The effects of gassing rate, impeller speed and electrolyte presence in the system have been examined.