This study addresses the general problem of multipurpose batch plants retrofitting. A two-stage methodology, previously developed in our laboratory, is proposed to solve the retrofit problem in which the AD-HOC discrete-event simulator is used to evaluate the different batch plant configurations proposed, at an upper level, by a genetic algorithm. In this paper, we will present the different steps of the algorithm. Our approach will be illustrated by an example inspired by Fuchino et al. (1994), a problem proposed by Bernal-Haro et al. (1997) and, finally, an industrial size one involving 10 products and 9 stages.