Public authorities are expected to facilitate industrial cluster development. Since competition between established petrochemical clusters is intense, a proactive approach is required. A quantitative method was developed to help a Regional Development Board identify promising new activities, chemical plants that would complement the existing regional cluster. This decision support tool was implemented as a prototype optimization procedure based on mass flows. A case study on the sub-cluster around the manufacture of PolyEthyleneTerephtalate demonstrated that the method discerns between options. Use of the tool on the location of new facilities thus enables a Regional Development Board to timely match its strategy with the decision process of a chemical company.