Kinetics of interchange between boric acid and tetrahydroxyborate has been studied by the band shape analysis of B-11 NMR signals in aqueous solution of pH 8-10 at 20-52-degrees-C and 0.1-250 MPa. The system was analyzed as two-site exchange B(OH)3 half arrow right over half arrow left B(OH)4-. The observed first-order rate constant toward the right (k(X)) is related to the second-order exchange rate constant (k(ex)) by the equation k(X) = k(ex)C(T)[OH-]/(k(b) + [OH-]), where k(b) = [B(OH)3]-[OH-]/[B(OH)4-], and C(T) is the total concentration of boron. Under the given conditions k(ex) is (2-7) X 10(6) dm3 mol-1 s-1 and DELTAH(double dagger), DELTAS(double dagger), and DELTAV(double dagger) are 20.1 +/- 1.0 kJ mol-1, -55.0 +/- 3.1 J mol-I K-1, and -9.9 +/- 0.5 CM3 mol-1, respectively. The activation parameters reflect the transition state forming dimeric [(OH)3B(mu-OH)B(OH)3]-. The results are compared with those of related reactions.