COMIS simulates multizone air flow and pollutant transport. It has been developed in an international context at Lawrence Berkeley National Laboratory (LBNL) and within International Energy Agency (IEA) Annex 23. At the end of Annex 23, the Swiss Federal Laboratories for Materials Testing and Research (EMPA) took over the co-ordination of further developments, and-together with Centre Scientifique et Technique du Batiment (CSTB)-the maintenance of COMIS. In January 2001, the "COMIS v3.1 with IISiBat v2.4" package was released, and is available now from CSTB. In this paper, the main features of the COMIS simulation engine, and the latest improvements are presented. An example demonstrates the use of COMIS. Pollutant transport and impact evaluation with the new version are discussed. The coupling of COMIS with the thermal simulation program TRNSYS is discussed.