Environmental isotopes (18O, 2H) have been measured, between March 1997 and March 1999, at 10 stations located in the Mediterranean seaside countries (2 in Spain, 1 in Tunisia, 7 in France). Data collected on monthly, event and fractionated basis allow to understand the modifications of the isotopic content of precipitation due to meteorological phenomena. A monthly monitoring shows that the Western Mediterranean has a unique isotopic characteristic that is between Atlantic and Eastern Mediterranean. A daily survey highlights the impact of the origins and trajectories of air masses on the amount and the oxygen-18 content of rainfalls: enriched and higher amount Mediterranean precipitation, depleted and lower amount Atlantic precipitation. At last, fractionated sampling of heavy precipitation allows to establish a typology of the variations of the isotopic composition during a rain event. ¿ 2001 American Geophysical Union