Fe(III) compounds FeCl3 and FeCl4- have been used as ESR spin probes in ionic liquids (ILs) at 293 and 77 K for the first time. They showed characteristic spectral patterns, which could be separated from each other by simulation. The largest contribution originates from aggregated FeCl4- and other exchange coupled species at g' approximate to 2.6. FeCl4- has been shown to be an identifiable, changeable, customizable, transferable, and extractable probe with contributions to a characteristic ESR fine structure. For simulation a spin Hamiltonian with up to fourth order and statistic distributions of spin coupling parameters has been used. The different Fe(III) signals coexist being dependent on the functionalization of the IL, i.e., on changing the chain length of the substituent at the imidazolium cation as well as varying the respective anion (BF4-, PF6-, Cl-, and FeCl4-). From the molecular structure and occurrence of the Fe(III) species conclusions could be drawn concerning their locations and reactions in polar and nonpolar compartments of the ILs. Their contributions could be purposefully adjusted via the molecular control of the properties of the ILs. The conversion of FeCl3 into FeCl4- and [FeCl4X2](3-) species could be observed to be dependent on the formation of polar and nonpolar domains in ILs.