The paper presents a proposal of a multi-model knowledge representation to be used within a retrofit methodology for chemical processes. The retrofit of an existing process is a complex and lengthy task. Therefore, a tool to support the steps of retrofit by reasoning about the existing process and the potential areas of improvement could be of great help. The use of structural, behavioural, functional and teleological models of units of equipment/devices of the process allows the designer to work with a combination of detailed and abstract information depending on the retrofit step. Our retrofit methodology consists of four steps: data extraction, analysis, modification and evaluation. The HYSYS ExtrAction Data (HEAD) and automatic hierarchical abstraction (AHA!) prototype systems have been implemented for the two initial steps. These systems have been applied to three case studies: the ammonia, acrylic acid and acetone processes. (C) 2004 Elsevier Ltd. All rights reserved.