A computer modeling tool is being developed for the detailed transient modeling of an Extravehicular Activity Atmospheric Control Subsystem (EVA ACS). An EVA ACS includes the astronaut, CO2 removal, moisture control, temperature control, and oxygen make-up components. This modeling tool will be used in trade studies evaluating competing components and subsystems to guide the selection and development of hardware for lunar and Martian missions.Several computerized modeling packages already exist, but no single program has all the capabilities needed. These capabilities include models of all EVA components on both steady-state and transient bases and sophisticated, general-purpose chemical process modeling capabilities. To meet these needs, the integrated modeling tool utilizes the Advanced System for Process Engineering (ASPEN) to perform pseudo-steady-state simulations, and the General Environmental Thermal Control and Life Support program (G189A) to handle overall control of the run and transient input output (I O), as well as transient modeling calculations and database functions (data tables and plots). These integrated runs are being performed in the Fortran language on a VAX 8830 system.An integrated run requires several preparatory steps. For both ASPEN and G189A, the appropriate component and physical property models, component and physical property data and calculation (component) sequences must be specified in a specially formatted input data file. For initial development runs, we selected a simplified astronaut model, CO2 adsorption on silver oxide, H2O adsorption on silica gel, and a noncondensing heat exchanger. Where there is no ASPEN system routine available to model a particular component, a user-written Fortran subroutine is added. Extra I O statements have been added to the programs to create and modify the interface data files that carry calculation results back and forth between ASPEN and G189A.