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°C and salinity S = 35. Considering stable boron isotopes 11B and 10B, 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. © 2001 Elsevier Science B.V.