Iodic acids are widely studied in atmospheric and biological applications but their inherent hydrophilic properties introduce complexities that affect their functionality and reactivity. We have shown that iodic acid (HIO3) dehydrates directly into iodine pentoxide (I2O5) in contradiction to the generally accepted multi-step dehydration mechanism where HIO3 dehydrates into HI3O8 first, then dehydrates into I2O5. The generally accepted mechanism is used to determine the concentration of iodic acid by TGA and is only valid for special conditions. The revised mechanism allows for the determination of concentrations of iodic acids under all conditions, and the more specific conditions where the accepted mechanism is valid are shown. The determination of concentration of iodic acid with the revised dehydration mechanism is dependent on assumptions of residual water and initial concentration of HI3O8. The validity of these assumptions is established by studying the absorption and hydration behavior of I2O5 from atmospheric water. These results will have an impact on the handling and use of iodine.