As the durations of manned space missions increase, so will the need for compact and reliable water recycling systems. Optimization of such water-recycling systems involves computer simulation of process elements and subsystems. The operations of water recycling systems are simulated at the Ames Research Center using commercial software called ASPEN-PLUS.Ion exchange is a part of the multifiltration subsystem, used for final polishing of recycled water and in some cases as a complete water treatment. Ion-exchange resins remove hazardous ions from solution by exchanging them with innocuous ions according to selection parameters. The ion-exchange operation is not provided in the ASPEN-PLUS multiprocess simulator package, but FORTRAN-callable modules may be added. Therefore we have adapted a FORTRAN program simulating multicomponent adsorption by ion-exchange resins, for use both as an ASPEN-callable module and as a free-standing simulator of the ion-exchange bed. Our model determines equilibria using a Pitzer model for activities of liquid-phase ions and resin-phase activities calculated using Wilson binary-interaction parameters.Parameters required by the simulation program are provided by laboratory experiments. The laboratory investigation also supplies performance measurements of resin beds for trapping hazardous ions.