This paper describes a dual-plane Electrical Resistance Tomography (ERT) system for measuring local air volume fraction and 3-D velocity distribution in gas-liquid flows. An implementation based on a so-called 'best-correlated pixels' method has been developed. Applying the 3-D velocity implementation to tomographic data sets obtained from an air-water swirling flow, axial, angular and radial velocity profiles have been successfully extracted. With the volume fraction distribution reconstructed using the "sensitivity theory based inverse solution using conjugate gradient" (SCG) method, the dispersed phase flow volumetric rate can be obtained. The experimental results obtained using the ERT system are in good agreement with the measurements obtained using a local probing technique.