Equilibria of periodate ion were reinvestigated in aqueous solution by using potentiometric titration, UV and Raman spectroscopies, and gravimetry simultaneously at 0.5 M ionic strength and at 25.0 +/- 0.2 degrees C. Stepwise acid dissociation constants of orthoperiodic acid were found to be pK(1) = 0.98 +/- 0.18, pK(2) = 7.42 +/- 0.03, and pK(3) = 10.99 +/- 0.02, as well as pK(2) = 7.55 +/- 0.04 and pK(3) = 11.25 +/- 0.03 in the presence of sodium nitrate and sodium perchlorate as background salts, respectively. pK(1) cannot be determined unambiguously from our experiments in the presence of sodium perchlorate. The molar absorptivity spectrum of H4IO6- and H3IO62- was determined in the range of 215-335 nm, as major species of periodate present from slightly acidic to slightly alkaline conditions. The solubility of periodate decreases significantly under alkaline conditions, and it was determined to be (2.8 +/- 0.4) mM by gravimetry, under our experimental conditions. None of these studies gave any clear evidence for an ortho-meta equilibrium and the frequently invoked dimerization of periodate. All measurements can quantitatively be described by the presence of orthoperiodic acid and its three successive deprotonation steps.