[en] Oxygen-17 magnetic resonance spectra of aqueous solutions of sodium arsenite were measured for 0.6 < [As(III)] < 4.1 m, 10 < t < 90⁰C. Line broadening of the water resonance in the 40 to 90⁰C range gives the rate of oxygen exchange between AsO(OH)₂^- and solvent water. Exchange takes place via a first-order pathway, ΔS₁/sup 2+/ = -120 +- 3 J K^-1 mol^-1, ΔH₁/sup 2+/ = 25 +- 1 kJ mol^-1, and a pathway second-order in arsenite, ΔS₂/sup 2+/ = -102 +- J K^-1 mol^-1, ΔH₂/sup 2+/ = 33 +- 4 kJ mol^-1. Rates were measured using solutions of pH approximately 10.2; exchange appears to be only slightly faster in solutions of lower pH