The transition metal iodates provide a group of compounds with centric and acentric structures and magnetic alignment at low temperature, optical absorption and emission, as well as piezoelectric, ferroelectric, and related properties, and possibilities of cross-interactions. Many inconsistent reports on these iodates and their hydrates exist, mainly due to lack of characterization. We report the occurrence of 13 compounds: Cr(IO 33; Mn(IO 3) 2; Fe(IO 3) 3; Co(IO 3) 2 and its 2 and 4 hydrates; Ni(IO 3) 2 in α and β forms, its dihydrate, and α and β tetrahydrates; Cu(IO 3) 2 and 3Cu(IO 3) 2 · 2H 2O (bellingerite). Some of these compounds are unstable in the presence of water but form metastably from solution. All but four of these compounds have now been prepared as single crystals and all have been characterized by differential thermal and thermogravimetric analyses, optical absorption from 35 to 33,000 cm -1, and in terms of refractive indexes and related optic parameters. In Part II structural, magnetic, and some nonlinear optical properties are reported.