A synthetic procedure based on thermal hydrolysis of iron(III) chloride solutions for the preparation of hematite (α-Fe2O3) and ferricoxychloride (FeOCl) colloids consisting of nanoparticles (NPs) is described. Transmission electron microscopy indicated that both colloids consisted of particles smaller than 10 nm. X-ray diffraction measurements revealed transformation of FeOCl NPs into α-Fe2O3 NPs after a few months. The phase transformation was explained in terms of redissolution – recrystallization process. UV-vis spectroscopy was used for precise determination of the band gap of α-Fe2O3.