Ships have been found to be major sources of air pollution in harbours. However, from January 1, 2010, a European Union directive requires that all ships at berth or anchorage in European harbours use fuels with a sulphur content of less than 0.1% by weight while previously, outside of Sulphur Emission Control Areas, up to 4.5% were allowed. The impact of this directive on air quality in some Mediterranean harbours was investigated based on observations made from August to October 2009 and 2010 at a monitoring station placed on a cruise ship, Costa Pacifica, following a fixed weekly route in the Western Mediterranean. The concentrations of SO2 were found to decrease significantly from 2009 to 2010 in three out of the four EU harbours; the average decrease of the daily mean concentrations in the different harbours was 66%. The decrease of SO2 was not statistically significant in the harbour of Barcelona because of large day-to-day variations, however measurements from monitoring stations in the harbour zone of Barcelona as well as downwind of the harbour of Palma de Mallorca confirm a strong decrease in the SO2 concentrations from 2009 to 2010. No decrease was observed in the non-EU harbour of Tunis. Neither NOx nor BC concentrations showed significant changes in any of the harbours ; JRC.H.2-Air and Climate