We determined apparent molar volumes V-phi at 298.15 <= (T/K) <= 368.15 and apparent molar heat capacities C-p,C-phi at 298.15 <= (T/K) <= 393.15 for aqueous solutions of HIO3 at molalities m from (0.015 to 1.0) mol.kg(-1), and of aqueous KIO3 at molalities m from (0.01 to 0.2) mol.kg(-1) at p = 0.35 MPa. We also determined V-phi at the same p and at 298.15 <= (T/K) <= 368.15 for aqueous solutions of KI at m from (0.015 to 7.5) mol.kg(-1). We determined C-p,C-phi at the same p and at 298.15 <= (T/K) <= 393.15 for aqueous solutions of KI at m from (0.015 to 5.5) mol.kg(-1), and for aqueous solutions of NaIO3 at m from (0.02 to 0.15) mol.kg(-1). Values of V-phi were determined from densities measured with a vibrating-tube densimeter, and values of CO were determined with a twin fixed-cell, differential temperature-scanning calorimeter. Empirical functions of m and T were fitted to our results for each compound. Values of K-a, Delta H-r(m), and Delta C-r(p,m) for the proton ionization reaction of aqueous HIO3 are calculated and discussed. (C) 2007 Elsevier Ltd. All rights reserved.