In this paper, we present the relative exergy array (REA), a tool to measure the relative exergetic efficiency and the controllability of a process when a proposed process and control structure is postulated. The REA is a means to compare thermodynamic efficiency for control structures of multi input-multi output processes. Although only steady-state information is employed, it is a tool for quick comparison between several candidate process control structures. Guidelines for interpreting the results obtained from the REA are presented and explained. A case study of dual composition control of a distillation column is presented and the results discussed.