A new approach to separation process synthesis and selection of single separations is presented. The method is based on the reuse of existing design cases by case-based reasoning (CBR). CBR is a method for finding the most similar existing separation designs and for applying the knowledge of their concept to solve new problems. The method has previously been applied for selecting single separations and simple sequences but has now been extended to cover synthesis of more complicated systems. The method is mainly intended to screening feasible process alternatives in preliminary process design for more detailed study by simulation. © 2005 Elsevier Ltd. All rights reserved.