Jet milling is often employed to produce very fine product or to tackle materials that are difficult to mill. This paper analyses particle breakage in a single jet region in a fluidised bed with a view to identify the role of jet hydrodynamics and material properties. By making a number of simplifying assumptions, the particle breakage in the jet can be estimated by coupling a hydrodynamic model of the jet with the kinetics of single-particle impact breakage. This approach works satisfactorily for a number of particulate solids, e.g., alpha-lactose monohydrate, but poorly for some others, e.g., paracetamol. The underlying cause of this feature is unclear, although it is likely to be due to the hydrodynamic mechanism. In this work, a sensitivity analysis has been carried out which highlights the importance of some of the hydrodynamic parameters on particle flow and breakage in the jet region. A more general model of jet hydrodynamics is needed to predict reliably the particle behaviour in this region. (C) 2004 Elsevier B.V. All rights reserved.