The bone stable oxygen isotope values and the distance of the stranding site from the innermost point of the estuary for thirteen marine mammal species (Otaria flavescens, Arctocephalus australis, Pontoporia blainvillei, Ziphius cavirostris, Phocoena dioptrica, Phocoena spinipinnis, Delphinus delphis, Globicephala melas, Grampus griseus, Lagenodelphis hosei, Orcinus orca, Pseudorca crassidens and Tursiops truncatus) from the Río de la Plata estuary and the adjoining western South Atlantic Ocean. Stable oxygen isotope values (δ18O) are reported relative to the Vienna Standard Mean Oceanic Water (V-SMOW).