Iron oxides are generally stable as a solid in solution phase, because they are barely soluble in water. However, they are formed by a precipitation reaction from a solution including precursor complexes of iron. This precipitation is thermodynamically treated using solubility products, such as aFe3+ (aOH-)3 (a is an activity). We must pay attention to this treatment, since this description is as a result of an ideal system. Apparently, so many complexes, including ones expressed in solubility products, are in equilibrium in the solution, from which the specific solid is precipitated. Precipitation reaction is regarded as a phase transformation like the freezing of water. In particular, for solidification of ionic crystal, a precursory complex is very important. Namely, the choice of a specific complex is a key step for the synthesis of a desired solid. For example, we cannot obtain the desired Fe2O3 particles if we just mix Fe 3+ and OH- ions stochiometrically in a solution, obeying 2Fe 3+ + 6OH→ Fe2O3 + 3H2O, because the solidification reaction includes the nucleation and the growth via supersaturation of precursor complexes. In other words, the identification of the precursor and the precise control of precursor concentration are very important. We can understand the mechanism of particle formation only when we discuss the precursory complexes. In the present chapter, iron complexes will be exemplified as a precursor for solidification of iron oxides. We will discuss the influence of coexistent ions on the formation of specific complexes and precipitation by way of UV-vis absorption spectrophotometry, and will also discuss the characterization of as-prepared solids by FT-IR spectroscopy.