Extractive or azeotropic distillations are commonly used to separate close or azeotropic boiling mixtures. An important consideration regarding the design of those separation units is to select suitable solvents. An useful selection methodology for mass separating agents is presented being realized in the system SOLPERT (Solvent Selecting Expert System) by means of combining databases, heuristic and numerical methods. Several inherent limitations of solvent screening methods, e. g. database search, empirical methods, and group contribution methods, are circumvented and minimized. SOLPERT is part of an environment of cooperating distributed expert systems, called PROSYN (Process Synthesis Manager), concerning computer aided process synthesis.