Dissolved boron in seawater occurs mainly in the form of boric acid (B(OH) 3 ) and borate (B(OH) 4 - ). While the equilibrium properties of the dissociation of boric acid have been studied in detail, very little work has focused on the kinetics of the boric acid-borate equilibrium in seawater. Here, we present a theoretical study of the relaxation of the seawater borate-carbonate system towards equilibrium using the experimental data of Mallo et al. [Nouv. J. Chim. 8 (1984) 373] and Waton et al. [J. Phys. Chem. 88 (1984) 3301]. The reaction rate constants are two to four orders of magnitude smaller than typical rate constants of diffusion-controlled reactions of other acid-base equilibria. This is presumably due to the substantial structural change that is involved in the conversion from planar B(OH) 3 to tetrahedral B(OH) 4 . The time required to establish the boric acid-borate equilibrium in seawater is calculated to be ~95μs at temperature T=25 o C and salinity S=35. Considering stable boron isotopes 1 1 B and 1 0 B, the isotopic equilibration time is ~125 μs. As a result, kinetic isotope effects during coprecipitation of boron in calcium carbonate are unlikely and therefore do not affect the use of stable boron isotopes as a paleo-pH recorder.