Multizonal models have been widely used for modelling the effects of mixing non-idealities in process equipment, presenting a realistic trade-off of computational efficiency and predictive accuracy between simple models based on idealised descriptions of mixing and full computational fluid dynamics (CFD) computations. However, a key weakness of multizonal models has been the difficulty of characterisation of the flow-rates between adjacent zones, and also of fluid mechanical quantities, such as the turbulent energy dissipation rate, that have important effects on the process behaviour within each zone. This paper presents a formal framework for addressing the above difficulties via a multiscale modelling approach based on hybrid multizonal/CFD models. The framework is applicable to systems where the fluid dynamics operate on a much faster time-scale than other phenomena, and can be described in terms of steady-state CFD computations involving a (pseudo) homogeneous fluid, the physical properties of which are relatively weak functions of intensive properties. Such processes include crystallisation and a wide variety of liquid-phase chemical and biological reactions. (C) 2003 Elsevier Ltd. All rights reserved.