This paper deals with the problem of finding, among many alternatives, the sequence of separation units that will isolate desired products from given feed at the minimum cost. Multicomponent products can be specified. An evolutionary procedure is presented. This method consists of two phases. In the first phase, a good initial structure is created by heuristic methods. In the second phase, the initial structure is successively modified by making evolutionary structural changes. This logic has been programmed on a digital computer and has been tested on several problems.