Air core formation has been investigated in hydrocyclones operated with clear water and a lucid suspension of glass balls. Hydrocyclones form a central air core which extends over the complete hydrocyclone length. Air is sucked in the core at the underflow discharge. The air core diameter can be determined balancing the positive pressure gradient and the centrifugal force in the rotational flow field. In dense flow separation (high feed solids content) the air core in the conical part of the hydrocyclone is suppressed. The hydrocyclone operates as it is air sealed because the solids are discharged trough the underflow as a rope. Then, air can be introduced to the hydrocyclone only on the feed side. In practice, feed suspension always contains more or less dissolved or dispersed air. Observations in a transparent hydrocyclone show that dissolved gas is released due to the pressure drop inside the hydrocyclone. The generated micro bubbles grow by coalescence and move in the centrifugal field toward the centre, where an air core is formed. (c) 2007 Elsevier Ltd. All rights reserved.