This paper presents a hybrid method for design of environmentally benign processes. The hybrid method integrates mathematical modelling with heuristic approaches to solving the optimisation problems related to separation process synthesis and solvent design and selection. A structured method of solution, which employs thermodynamic insights to reduce the complexity and size of the mathematical problem by eliminating redundant alternatives, has been developed for the hybrid method. Separation process synthesis and design problems related to the removal of a chemical species from process streams because of environmental constraints are particularly suited for solution with the hybrid method. Application of the hybrid method is highlighted through two illustrative examples. The first example involves the determination of an optimal flowsheet for the removal of a chemical species from an azeotropic mixture and the second example involves the determination of environmentally benign substitute solvents for removal of a chemical species from wastewater. (C) 1999 Elsevier Science Ltd. All rights reserved.