[en] The Rio de la Plata estuary is the collector of a vast drainage basin of about 3.1 x 10⁶ km². Their major tributaries are the Parana, Paraguay and Uruguay rivers. The main course of the Parana river starts at the 'Planalto do Brasil', located at the central east Brazil. Pilcomayo and Bermejo rivers contribute to Parana water from the high Andes range. Paraguay river release water coming from the Mato Grosso. The Uruguay river, directly and the Iguazu river through the Parana, also contribute water from Brazilian east coastal hills to the estuary. Less significant are other rivers like the Salado, Negro, etc. The objective of this work is to study the variations in isotopic composition of the Rio de la Plata estuary and its correlation with the ITCZ, ENSO-related, and other meteorological phenomena. The methodology consists in the monthly collection of a 2 liter sample in the estuary and the measurement of ¹⁸O and ²H concentration. An aliquot is reserved for further tritium analysis, mainly related to nuclear plants on the Parana river. In this report, we present 40 isotope analyses for the Rio de la Plata estuary. Cyclical variations of oxygen isotopes, with minima in winter and maxima in summer has been observed, ²H concentrations also change cyclically. Figure 1 shows the ¹⁸O variation that is influenced by the austral summer ITCZ that migrates to higher southern latitudes leading to an increased pluviosity and thus to an amount effect. (Albero and Panarello, 1982; Rozansky and Araguas Araguas (1995). The signal in the estuary is delayed in about 4 months due to the average transit time of water from the catching areas. The deuterium composition follows also a cyclical pattern but is strongly influenced by ENSO-related phenomena, 'El Nino', which increases the ocean surface temperature, leading to an enhanced kinetic effect, that increase the 'd' value and 'La Nina (cold episode) making the evaporation process occurs more close to isotopic equilibrium, resulting in a smaller deuterium excess