ECOSIM is a software tool for the simulation of Environmental Control and Life Support (ECLS) systems which has been developed for the European Space Agency.A preliminary model of the Hermes Air Management System has been developed during the ECOSIM testing in order to assess the functionality of the software and to verify its results with those obtained from previous simulation tools.The model represents the Hermes cabin with its crew and it includes submodels for the sub-systems performing the following functions:Temperature and Humidity Control. Total Pressure and Composition Control. Air revitalisation.The interactions between these different subsystem are taken into account by the model, while many of the previous simulations made assumptions to decouple the different subsystems (e.g: a constant cabin temperature has been assumed during cabin depressurization transients, to decouple the pressure control section from the air conditioning section). The model allows to check the validity of these kind of assumptions.The paper also presents the results obtained with this model for the following experiments:Normal 24 hour cycle. Re-entry. Depressurization due to a micro-meteorite hole. Failure of the condensate water separators.The first version of ECOSIM will be released to the space industry before the end of year 1992 and the user's feedback will lead to improvements being implemented in later versions.