The presence of electrolytes in aqueous solutions has long been recognized as contributing to significant departures from thermodynamic ideality. The presence of ions in process streams can greatly add to the difficulty of predicting process behavior. The difficulties are increased as temperatures and pressures within a process are elevated. Because many chemical companies now model their processes with chemical process simulators it is important that such codes be able to accurately model electrolyte behavior under a variety of conditions. Here we examine the electrolyte modeling capability of ASPEN PLUS™, a widely used simulator. Specifically, we will present our efforts to model alkali metal halide and sulfate systems. We will show conditions for which the models within the code work adequately and how they might be improved for conditions where the simulator models fail.