An experiment was carried out in a pilot-size aerated mixing vessel using dual Rushton impellers. The two-phase mixture was composed of air and deionized water. Different gas-filled cavity structures were recognized close to the outer edge of the impeller blade by frequency transformation of the time-domain structural function. The majority of hydrodynamic regimes for the upper impeller were of VC structure, while the lower impeller operated over the VC, 1L, 2L, S33, L33, and RC structures respectively. Local void fraction alpha was also measured by a resistivity probe at 250 modes in the vertical half-section plane of the vessel. Spatial distributions of alpha are presented for the aforementioned hydrodynamics regimes. A transition from quite homogenous local void fraction distribution in a VC structure to a rather nonhomogeneous void distribution with a high concentration of bubbles around the shaft was detected.