The carbon dioxide content of the atmosphere [measured as the partial pressure of CO2 (pCO2)] affects the content of the surface ocean, which in turn affects seawater pH. The boron isotope composition (delta11B) of contemporaneous planktonic foraminifera that calcified their tests at different water depths can be used to reconstruct the pH-depth profile of ancient seawater. Construction of a pH profile for the middle Eocene tropical Pacific Ocean shows that atmospheric pCO2 was probably similar to modern concentrations or slightly higher.