IEA-ECB&CS Annex 23 program intends to provide a validated and user-friendly computer code simulating air and contaminant flows in multizone buildings. When developing such a code, which intends to be a model of the reality, it is essential to check, at each step, its conformity whith the model. At the other end, the claimed user-friendliness should also be checked with real users. This evaluation task took a large part of the efforts and means put into Annex 23 by participating countries. The result is that COMIS was checked very carefully, using up-to-date strategies and tools. This report presents the methods used and the results of this huge validation and user test strategy, methods and computer tools for sensitivity analysis of the code. Simulation results were compared whit more than 50 simple benchmarks or test cases, for which either an analytical or a numerical solution was obtained using classical tools (such as Excel or Mathematica). Each of these test cases was created to check a particular feature of COMIS. This so-called analytical evaluation allowed the correction of several bugs which appeared in the early versions of COMIS.