This paper proposes an approach for obtaining reduced-order models of chemical processes with application to the design of plantwide control systems. The approach is based on the inherent structure that exists in a chemical plant and identifies units or groups of units which determine the steady-state and dynamic behaviour of the plant. Specific reduction techniques with different accuracy are applied to each unit, followed by the coupling of the reduced models, according to the process flowsheet structure. By means of a case study, the assessment of plantwide control structures for the iso-butane-butene alkylation plant, the effectiveness of the approach is proven. Moreover, it is shown that the approach is able to retain the nonlinearity of the original plant model and preserves the significance of important model variables. (C) 2008 Elsevier Ltd. All rights reserved.