Ventilated gas cavities were observed when air was sparged directly to the rear of one blade of a multi-bladed disc-turbine impeller. For a constant gas rate, it was found that for low impeller speeds no cavities were formed. As the impeller speed was increased, large, tapered cavities were formed and increased in length with increasing impeller speed. At a critical speed, however, the cavities reached a maximum length, beyond which the cavity length decreased with increasing impeller speed. For each gas ventilation rate, it was found that the internal pressure of the cavity decreased only slightly for speeds up to the corresponding critical value. Beyond this point the internal pressure decreased significantly with further increases in impeller speeds. A gas flow model was developed based on the assumption that the gas exited the cavity through an opening (orifice) located randomly on the gas-liquid interface.