Abstract At low pH values and in the presence of excess peroxide, lactoperoxidase and horseradish peroxidase will catalyze the stoichiometric oxidation of iodide to iodine. Neither enzyme oxidizes iodine to iodate. These peroxidases can be used, therefore, to prepare pure I 2 solutions. This has made possible a reassessment of the extinction coefficients of aqueous I 2 and I 3 −, and of the iodine-triiodide equilibrium constant. Millimolar extinction coefficients for I 3 − at 350 nm and 460 nm are 21.0 and 0.87, respectively. Millimolar extinction coefficients for I 2 at the same wavelengths are 0.035 and 0.70. A value of 768 M −1 was obtained for the iodine-triiodide equilibrium constant at 24°C.