An experimental Study was carried out on the volume of the cavity formed behind impeller blades in aerated agitated vessels with highly viscous liquids. Two Rushton turbine impellers with different sizes were used to measure the volume of the "stable cavity" that remained behind impeller blades after stopping aeration. Agitation power was also measured under the condition of stable cavity formation and compared with that required for normal operation of an aerated agitated vessel. From the results, the relationships between the volume of the cavity formed in the normal operation and that of the stable cavity were considered. Stable cavity Volumes were measured over wide ranges of agitation speed and test liquid viscosity. The results were discussed in terms of the forces acting on the cavity, and correlation equations for the cavity volume were derived for both laminar and turbulent flow regions.