Recent studies have used the Ba/Ca ratio of planktic foraminifer shells as a proxy for river run-off at oceanic sites near estuaries. Such studies assume that the Ba/Ca ratio in planktic foraminifer shells is primarily controlled by the Ba/Ca concentration of seawater and that other parameters such as salinity, temperature and pH do not compromise the primary Ba concentration relationship. Here we provide new insights from culture experiments and review published studies to confirm that environmental parameters including pH, temperature, salinity, and symbiont photosynthesis do not affect Ba substitution into planktic foraminiferal calcite. The partition coefficient for Ba in spinose planktic foraminifers is estimated as DBa=0.15±0.05 (95% confidence limits). The same factor also seems applicable to the non-spinose genus Neogloboquadrina but not to specimens of the non-spinose genus Globorotalia. © 2011 Elsevier B.V.