A generic System for On-Line Optimization (SOLO) has been developed and implemented in a Benzene plant at the manufacturing sits of Dow Benelux N.V. in Temeuzen. The system is characterized by an integrated detailed modular structure involving fundamental and state-of-the-art model-based technology. It exploits a large set of analog real-time data. Digital real-time data is retrieved as well, representing The plant control activities and process conditions. Consequently, the system takes into account essential plant-operational aspects to obtain true plant-compatibility. This being recognized as a main characteristic of the underlying system, is a requirement for successful implementation. Not only a continuous economical, but a reliable and safe operational mode is thus being achieved within the defined constraints. The system features generic modules for data-retrieval, data-analysis, data-reconciliation and decision mechanisms. In addition, the plant-specific modules for parameter-estimation and the final optimization were developed. All constituent parts and the various interfaces are integrated and controlled by a main executive, supported by an in-house developed menu- & mouse-driven graphical operator interface. The solving engine is Speedup, a flowsheet- & equation-based tool for steady state and dynamic simulation with a multipurpose external interface. The tool originates from Imperial College London and is currently a trademark of AspenTech Inc. A thorough validation preceded the final implementation of SOLO in June ’92, increasing the plants profit-margin by an average of 4 % (quantified on-line). Additional benefits were achieved by the implementation of a revamp proposal, generated from an occasional design optimization study involving the available plant model.