The air core is one of the most important internal structures in the flow field of hydrocyclones. One difficulty in predicting the flow split in cyclones is the inability to specify the form and location of the air-core surface. In the usual cyclone models the interface that bounds the air-core is described by a fixed cylindrical surface, which greatly simplifies the problem. Most of the available air core models suit only hydrocyclones operating with water or very dilute slurries. An attempt has been made in this study to develop a semi-empirical model, capable of predicting the air core diameter correctly for hydrocyclones under different operating conditions. The model considers the effect of axial length, underflow and overflow dimensions, feed solids concentration and other significant operating conditions on predict air core size. The predictions are compared with data sets available in the literature. It was observed that the air core diameter reduces with increase in the feed solids concentration and with decrease in height from the roof of the cyclone towards the apex zone. © 2011 Elsevier B.V. All rights reserved.