Multizonal models and, in particular hybrid multizonal/CFD models, represent a powerful approach to process simulation when complex physical and chemical phenomena (e.g. crystallisation, polymerisation, bioreactions) need describing by taking into account mixing and other fluid flow properties. A major issue in setting up such models is the definition of a suitable network of zones. This paper addresses this issue by delivering some criteria to establish a suitable network of zones. The suggested procedure will be compared by means of a mixing example. (C) 2003 Elsevier Ltd. All rights reserved.