Within Annex 23 (＇Multizone Air Flow Modelling＇) of the LEA (International Energy Agency), a large part of the effort went to the evaluation of the developed simulation tool COMIS [J.-M. Furbringer, C.-A. Roulet, R. Borchiellini, Evaluation of COMIS, IEA Annex 23: Multizone Air Flow Modelling, LESO-EPFL, Lausanne, 1996; J.-M. Furbringer, C.-A. Roulet, R. Borchiellini, Evaluation of COMIS -appendices, IEA Annex 23: Multizone Air Flow Modelling, LESO-EPFL, Lausanne, 1996]. The purpose of this work was to examine the reliability of the output of the program and also the impact of the input parameters on the simulation result. Therefore, the program was tested in different ways, for several cases with sometimes strongly different input parameters. Two cases were examined in detail by the BBRI. (Belgian Building Research Institute): Spread of contaminants in an apartment with natural ventilation: this case will be discussed in this text. Single-sided ventilation in a PASSYS-test cell. Thanks to all these different simulations, one can have an increased confidence in the simulation tool COMIS. More exactly, it gives an idea of the possibilities and limitations of the tool.