Successful pneumatic conveying of granular materials can be achieved only when: the air mass flow rate is controlled within a certain range; the pipe size is selected appropriately; and the operating boundaries are well assessed. This paper presents results from investigations into the capacity limitation of low-velocity slug-flow pneumatic conveying. The mechanism for the formation of the unstable zone also is explored experimentally and theoretically. A new theoretical model based on observed unstable flow mechanisms and stability criteria is presented for the purpose of predicting transport boundaries. The modelling results agree very well with the experimental data obtained on poly pellets and different sizes of conveying pipeline. (C) 2003 Elsevier Science B.V. All rights reserved.