The recent trend in the chemical industry is the rapid growth in the production of specialty chemicals, that is, specialty monomers, agricultural chemicals, pharmaceuticals, and so forth (high-value-added materials). The products produced often are very large molecules, and normally there are a very large number of reaction paths that can be used to produce them. Another feature of these multistep reaction problems is that the reaction chemistry usually is not completely known (a single reaction having a yield of less than unity is given or a single reaction that is not stoichiometrically balanced is reported for each reaction step). A first screening of these reaction paths can be accomplished by introducing fictitious reactions and/or fictitious components to account for the chemical uncertainties, and then using these approximations to calculate the raw materials costs and waste costs. Because the chemistry is not complete and the waste loads do not include separation system waste costs, all reaction paths within about 50% of the best should be retained for further consideration.