To date, almost all water used by the crew during space flight has been transported from Earth or generated in-flight as a by product of fuel cells and used for relatively short periods. With the United States' commitment to a permanent manned presence in space, this is no longer adequate. In order to resolve this problem, a plan for nearly complete recovery and recycling of water on-orbit has been formulated. Closed-loop water recovery on a scale applicable to a manned space station was first studied in the late 1960s and early 1970s. These tests, conducted with breadboard and early prototype equipment, demonstrated the feasibility of sustaining humans in a closed environment by a regenerative Environmental Control and Life Support System (ECLSS). However, increased awareness of potential hazards associated with long-term reuse of reclaimed water has resulted in the recognition that additional characterization of closed-loop water recovery systems and products is essential.A testing program has been developed at the Marshall Space Flight Center (MSFC) to further investigate the operational aspects of a closed loop water recovery system. In order to support the various operational scenarios involved with the testing activities, a computer model of the system has been developed using the Computer Aided Systems Engineering and Analysis program (CASE/A). The model will be used to predict various attributes related to the test protocol including process mode, mass balance, tank quantities, tank modes, water availability, and water purity (correlation of water cleanliness to the number of times it has been processed). The model will be used preceding testing operations in order to determine problems associated with the test protocol. After the tests have been completed, results from the model will be coupled with analytical results from the chemistry lab (water chemical composition) to determine specific correlations between transient test operational parameters.