Culture experiments were carried out with the species Orbulina universa at four different pH values (7.70¿0.05, 8.15¿0.05, 8.60¿0.05, and 9.00¿0.10) in order to establish the pH-dependence of boron isotope fractionation between seawater and foraminifera. A clear relationship between the boron isotopic composition of the foraminifera and the pH of the seawater culture solutions was found, showing heavier boron isotopic composition at higher pH. This finding supports the viability of boron isotopes as a paleo-pH tool. It is important to note that Orbulina universa cultured in natural seawater, as well as those obtained from coretop samples, have a significantly lighter boron isotopic ratio than Globigerinoides sacculifer from coretop samples, suggesting that at least for this species, a vital effect is active. ¿ American Geophysical Union 1996