Mitsubishi Kasei and Mitsubishi Petrochemical merged to become Mitsubishi Chemical in 1994 making it the largest chemical company in Japan. This allows two original companies' production sites, especially for those are physically built next to each other, to improve themselves by multi-site integration. Mitsubishi Yokkaichi production plant site is one of the typical examples, which consists of three individual plant sites in the old company structure. Each plant site contains a utility plant to generate steam and electricity for chemical production. After the company merge, connecting steam and electricity among plant sites have been carried out for better flexibility, efficiency as well as capability of the utility systems. Besides these, there are still many other improvement alternatives. A site-model, which includes all three utility plants and production units, was then developed to explore further opportunities. In this paper, applications at Mitsubishi Yokkaichi production site are presented to illustrate the features of the sitemodel. (C) 2003 Elsevier Ltd. All rights reserved.