A multi-element fixed control volume integrated air interchange system performance computer model has been developed and upgraded for the evaluation/assessment of atmospheric characteristics inside the crew compartments of the mated Orbiter and International Space Station (ISS). In order to ensure a safe, comfortable, and habitable environment for all the astronauts during the Orbiter/ISS docked period, this model was utilized to conduct the analysis for supporting the early ISS assembly missions. Two ISS assembly missions #2A and #4A were selected and analyzed. This integrated system analysis utilizes the available ISS flight plan mission timeline information to assess interchange airflow supply capabilities, develop integrated air interchange system limitations and requirements, evaluate each air parameters within each crew cabin, and determine any possible system operating constrains to ensure the mated system performance will meet both the specific environment and interface hardware design requirements. The integrated system performance analysis/assessment results for these two early ISS assembly missions are presented to show the capabilities of this model can provide a valuable design and operating information to support all the planned ISS assembly missions.