This paper presents a simultaneous optimization method for the retrofit of existing mass-exchange networks (MENs). A mixed-integer nonlinear programming (MINLP) model is formulated on the basis of a stage-wise superstructure representation for the MENs. The MINLP formulation simultaneously considers the costs of mass separating agents, the reassignment of existing exchange units to different process stream matches and their need for additional tray numbers, column height, or both, and the creation and installation of new units. Several examples from the literatures are supplied to demonstrate the applicability of the proposed MENs retrofit model.