Secondary aerosol produced from marine biogenic sources in algal-rich coastal locations will initially be composed of iodine oxide species, most likely I<sub>2</sub>O<sub>5</sub>, or its hydrated form HIO<sub>3</sub>, formed as a result of iodine gas-phase chemistry. At present, there is no quantitative hygroscopic data for these compounds and very little data available for iodate solutions (HIO<sub>3</sub> and I<sub>2</sub>O<sub>5</sub> share a common aqueous phase). With increased interest in the role of such aerosol in the marine atmosphere, we have conducted studies of (i) the deliquescence behaviour of crystalline HIO<sub>3</sub> and I<sub>2</sub>O<sub>5</sub> at 273–303 K, (ii) the efflorescence behaviour of aqueous iodate solution droplets, and (iii) properties (water activity, density, and viscosity) of subsaturated and saturated iodate solutions. <br><br> The deliquescence of I<sub>2</sub>O<sub>5</sub> crystals at 293 K was observed to occur at a relative humidity (DRH) of 80.8±1.0%, whereas for HIO<sub>3</sub>, a DRH of 85.0±1.0% was measured. These values are consistent with measured water activity values for saturated I<sub>2</sub>O<sub>5</sub> and HIO<sub>3</sub> solutions at 293 K of 0.80±0.01 and 0.84±0.01 respectively. At all temperatures, DRH values for HIO<sub>3</sub> crystals were observed to be higher than for those of I<sub>2</sub>O<sub>5</sub>. The temperature-dependent DRH data, along with solubility and water activity data were used to evaluate the enthalpy of solution (ΔH<sub>sol</sub>) for HIO<sub>3</sub> and I<sub>2</sub>O<sub>5</sub>. A ΔH<sub>sol</sub> value of 8.3±0.7 kJ mol<sup>−1</sup> was determined for HIO<sub>3</sub> which is consistent with a literature value of 8.8 kJ mol<sup>−1</sup>. For I<sub>2</sub>O<sub>5</sub>, we report for the first time its solubility at various temperatures and ΔH<sub>sol</sub> = 12.4±0.6 kJ mol<sup>−1</sup>. The measured water activity values confirm that aqueous iodate solutions are strongly non-ideal, consistent with previous reports of complex ion formation and molecular aggregation.