A collaborative research project for air pollution reduction was set up in 2006 between the Joint Research Centre of the European Commission (JRC) and the Authorities of the Lombardy Region in Italy. Among other research activities the PO valley Model Intercomparison exercise (POMI) has been carried out in order to explore the changes in air-quality in response to changes in emissions. The starting point was the evaluation of the simulated particulate matter and ozone against observations for the year 2005 of the six participating Chemical Transport Models? (CTM). As models were run with the same configuration in terms of spatial resolution, boundary condition, emissions and meteorology, the differences presented in the models? results should only be related to their formulation. As described in the paper much effort has been put to improve the accuracy of emissions and meteorology. Nevertheless none of the models using the proposed meteorology succeeded to fulfill the performance criteria set in the 2008 Air Quality Directive and in the literature for particulate matter, while also for ozone the results are not very satisfying. Possible explanations for this common behavior and a discussion of the differences among models? results are presented.