Stable isotopes have been widely used in the literature both to discuss current ocean circulation processes, as well as to reconstitute paleoceanographic parameters. the distribution of oxygen and deuterium stable isotopes in seawater (δ 18 O sw and δD sw) at the Western tropical South Atlantic border was investigated to better understand the main fractionation processes of these isotopes and establish a regional salinity and δ 18 O sw relation to improve the paleoceanographic knowledge in the region. this study was conducted during a quasi-synoptic oceanographic cruise in which 98 discrete seawater samples were collected in the core of the main water masses for stable isotope analysis. A strong correlation between δ 18 O sw and δD was found, which made it possible to extrapolate the results for δ 18 O sw to δD. Although it was not possible to distinguish the water masses based only on their isotopic signatures, the water masses had a strong salinity and δ 18 O sw relation, and compared with previous studies, a seasonal pattern was observed. Paleosalinity differences of up to 0.2 psu between Summer and Winter are reported. Considering the limitations of the current techniques to seasonally separate the samples for the paleoceanographic studies, an intermediate Mixing Line for the Tropical South Atlantic (SSS = 1.942* δ 18 O sw + 34.56) was proposed to reduce the estimated errors associated with these seasonal variations.