Rate constants for the hydration and dehydration reactions of carbon dioxide with water and with hydroxyl ion were measured in seawater by a pH‐stat method at salinities (× 103) from 3.4 to 37.06 at 25°C and from 5° to 35°C at a salinity of 33.77. Transition state theory was used to extrapolate the rate constants to infinite dilution, with the following results at 25°C: kco2 = 0.037 ± 0.002·s−1, koh‐Kw = (7.1 ± 0.7) × 10−11 mol·dm−3·s−1, kd = (7.6 ± 1.1) × 104 dm3·mol−1·s−1, and khco3− = (1.8 ± 0.3) x 10−4·s−1. The corresponding values in seawater of 33.77 salinity and 25°C calculated from equations fit to the experimental data are 0.036, 13.5×10−11, 3.5 × 104, and 1.40 × 10−4.