In many complex monitoring and diagnostic applications it is useful to build separate expert systems for different subsystems and have them operate (on different computers) mostly in an independent mode with only occasional interactions. There are many advantages of such a scheme including easier knowledge engineering and increased overall system performance. Using an enhanced version of the Lockheed Expert System (LES), we have developed a communicating expert systems for fault diagnosis and fault correction in a prototype for the Space Station Air Revitalization System (ARS). The system consists of three communicating expert systems, one for oxygen generation, one for CO2 removal and a supervisor for overall control. The three expert system modules communicate via mailboxes. The purpose of this work is to gain an understanding of the problems involved and advantages of using such a communicating expert systems framework.