The Environmental Control and Life Support System (ECLSS) for both MPLM and APM is developed by Dornier. One of the main ECLSS functions to be realized is the atmosphere pressure control of the module. It includes the Cabin Depressurisation, the Positive Pressure Relief, and the Negative Pressure Relief. In order to verify the functionality of the atmosphere pressure control system (ACS) analyses of different scenarios have to be performed. The analysis method including the governing equations of the three ACS functions is described. The analyses are performed with a Dornier developed software program that solves the set of equations numerically. The results of the analyses in terms of pressure and temperature transients are presented and discussed with respect to the boundary conditions and the influence of humidity.