Factors intrinsic to foraminiferal tests result in different degrees of susceptibility to dissolution. Differential resistance to dissolution among planktonic foraminifera has been studied in several regions, but no previous work has been reported for the western South Atlantic. The goal of this research was to develop a dissolution susceptibility ranking for planktonic foraminifera from the western South Atlantic Ocean, to compare the solubility between benthic and planktonic foraminifera, and to evaluate changes in the oxygen isotopic signal (δ18O) associated with dissolution. Two experiments were carried out by immersing tests in acetic acid or distilled water for 200 days. Our comparison revealed that tests of planktonic foraminifera were more resistant to dissolution than the benthic species tested, which has implications for use of the planktonic/benthic ratio (P/B) as a preservation proxy. Solubility of tests is directly proportional to the Mg content in the calcite structure, which varies widely across benthic taxa but is consistently low in planktonics. The δ18O increased during dissolution, probably due to the preferential dissolution of the internal chambers, making the remaining calcite progressively reflect the composition of the last chambers. Our solubility ranking for planktonic foraminifera agreed in part with rankings for foraminifera from other regions. Neogloboquadrina pachyderma and Globorotalia inflata proved to be resistant species; Globigerinella calida, Globigerinella siphonifera, Globorotalia hirsuta, Candeina nitida, and Trilobatus trilobus were moderately resistant; and Globorotalia fimbriata and Orbulina universa were most susceptible to dissolution. Presence and condition of benthic genera such as Quinqueloculina, Hoeglundina and Bulimina, and the planktonic species O. universa can be used to indicate limited dissolution in paleoceanographic studies. Small variations in the dissolution index may also reflect oceanographic changes that influence the composition of the tests, indicating the importance of regional solubility rankings, such as this study for the western South Atlantic.