The solubility studies of lanthanoid iodates in aqueous and aqueous-organic solvent mixtures were reviewed. The solubility data of the iodates in water were compiled and critically evaluated. The preparation methods of crystalline lanthanoid iodate dihydrates, Ln(IO₃)₃.2H₂O (Ln = Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Lu), and amorphous lanthanoid iodate anhydrides, Ln(IO₃)₃ (Ln = Nd and Sm), were described in details. The results of solubility measurement of the crystalline dihydrates and the amorphous anhydrides of neodymium and samarium iodates were discussed. We found that during the solubility measurement the amorphous iodate anhydrides prepared by thermal dehydration recrystallized to the crystalline iodate dihydrates. This fact was supported by the results of X-ray diffraction analysis and IR spectoroscopy. A summary of the evaluations for the solubility of lanthanoid iodates, LN(IO₃)₃ (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Lu), in water was given. From the solubilities measured in our studies and the published data, the recommended solubility values of some lanthanoid iodates, Ln(IO₃)₃ (Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Tm), in water at 298.2K were decided by applying the guideline of the IUPAC Solubility Data Project The recommended solubilities were tabulated. The reported solubilities of lanthanum, cerium and lutetium iodates were designated as tentative values. The theoretical treatment of electrolyte solution reported by Abe was summarized. Applied the theoretical equation, the solubilities of sparingly soluble lanthanoid iodates in dimethyl sulfoxide-water mixed solvents were successfully estimated. The change in solubility of the lanthanoid iodates in aqueous and aqueous-organic solvent mixtures as a function of ionic radius of lanthanoid ions or lanthanoid atomic number is also discussed.