Abstract The method most often used by process designers to solve the formidable task of synthesizing a process flowsheet is probably the evolutionary approach. The method is easily based on the previous experience of the designer and involves moving to better and better flow-sheets by making a succession of small improvements to an existing one. This paper presents a first step to organize the evotutionary synthesis of process flowsheets. In general terms it discusses the rules to make modifications and their desired properties, strategies to use these rules, and the various means to compare flowsheets. Specific rules and strategies are then proposed and successfully demonstrated on the evolutionary synthesis of multicomponent separation flowsheets. Where appropriate, proofs are given that these specific modification have the stated desired properties.