The reactivity of hypoiodous acid (HOI) is an important factor for the fate of iodine in oxidative drinking water treatment. The possible reactions of HOI are its disproportionation, its oxidation to iodate (IO3-), or the reaction with natural organic matter (NOM). The latter reaction may result in the formation of iodoorganic compounds which are frequently responsible for taste and odor problems. The acid dissociation constant (pK(a)) of HOI has been determined spectrophotometrically as 10.4 +/- 0.1 (T = 25 degrees C; I = 50 mM). Kinetic constants and a new rate law for the disproportionation of HOI as catalyzed by hydrogencarbonate, carbonate, and berate are presented. In the pH range 7.6- 11.1, the main uncatalyzed reactions are HOI + HOI (k(1) = 0.3 M-1 s(-1)) and HOI + OI- (k(2) = 15 M-1 s(-1)). The buffer-catalyzed reaction step was found to be second-order in HOI and first-order in the buffer anion. The following rate constants were deduced: HOI + HOI + HCO3-: 50 M-2 s(-1): HOI + HOI + CO32-: 5000 M-2 s(-1); HOI + HOI + B(OH)(4)(-): 1700 M-2 s(-1). All these rate constants result in half-lifes for HOI of 10-1000 days under typical drinking water conditions. (C) 2000 Elsevier Science Ltd. All rights reserved.