Manned Spacecraft are equipped with an Environmental Control Subsystem which assures that an environment is provided in which the crew finds optimum conditions to work in nominal conditions. Furthermore, in case of emergency this subsystem has to assure survivable conditions for the crew.One of the most important functions of an Environmental Control and Life Support Subsystem is to control the pressure and composition of the atmosphere. The pressure of the atmosphere is influenced by temperature changes, consumption of one or more constituents of the atmosphere by crew metabolism or other thermophysical processes and by eventually occurring gas losses due to leaks or punctures of the structure.In the paper presented a simulation model is described, which allows to analyse the thermophsyical behaviour of the Atmosphere Pressure Control Section of an ECLSS under simultaneous changing boundary conditions. The HERMES Spaceplane is taken as an example to show the above mentioned effects on the pressure of the atmosphere in the space vehicle. It is shown that the influence of temperature change and eventual gas losses is high compared to the effect of crew metabolism. The simulation model is used to check the function of the N2-pressure and O2-partial pressure control equipment against the prevailing requirements for normal and emergency mode.