Teeth and otoliths are metabolically inert structures that preserve a chronology of chemical variations that may be related to the environmental histories experienced by each organism. Because of the natural decrease of barium and increase of strontium bioavailability in water with increasing salinity, these elements may be especially useful to track habitat use in aquatic organisms. Therefore, we tested whether the Ba/Ca and Sr/Ca ratios in dolphins' teeth represent a salinity gradient. The main aim was to determine whether these elements can be used as a natural tag for different aquatic environments. Teeth from two freshwater dolphins (Inia geoffrensis and Sotalia fluviatilis) and two marine species (S. guianensis and Pontoporia blainvillei) from Brazil and Uruguay were analyzed using Laser Ablation Inductively Coupled Plasma-Mass Spectrometer. Intensity ratios of 138Ba/43Ca and86Sr/43Ca were measured along a line that covered all growth increments in the dentin from the second year of life onwards. Teeth from the freshwater species had mean Ba/Ca values tenfold higher than marine dolphins, confirming the inverse relationship between salinity (and thus ambient Ba/Ca) and teeth ratios. Furthermore, Ba/Ca ratios could also differentiate dolphins from lower-salinity estuarine areas from those of areas with minimal freshwater discharge. No significant differences were found for Sr/Ca values. Results presented encouraging indications for the application of this technique as a potential new tool for studying habitat use in aquatic mammals.