s u m m a r y Salinization in coastal areas of the province of Buenos Aires, Argentina, is a controlling factor of water quality, and limits its exploitation for human uses and local ecological values. Coastal plains such as the one in the southern part of Samborombón Bay have a tidal morphology consisting of fine sediment deposits, tidal channels, and aeolian sand sheets, each of them playing a hydrological role. In this paper, we describe the hydrological processes in the Samborombón Bay coastal plain using hydrochemical and isotopical data to provide evidence of salinization processes. Specific ionic relationships for surface and ground water samples from each formation are differ from those of sea water. Isotopic data also reveal infrequent participation of sea water in the coastal area water samples, showing an evaporation trend in both surface and ground water. We conclude that salinization has its origin in the occurrence of gyp-sum and halite, as well as other chlorides, from the coastal plain deposits. Rainfall recharge, continental water inputs, evaporation and long residence times define the hydrochemical and isotopical features of ground water, which shows no signs of present sea water contribution. Sand sheets constitute local aqui-fers with lower salinity content. Continental surface waters, as well as rainfall drainage via tidal channels, have a dominant effect on surface water hydrochemistry notwithstanding tidal oscillations. Exploitation of water resources is thus limited to regionally extended aeolian sand sheet formations and to surface waters, specifically those related to contributions of freshwater discharge from inland areas.