A series of paramagnetic ionic liquids, which are comprised of 1-alkyl-3-methylimidazolium RMI⁺ cation (R = Et, n-Bu, n-hexyl and n-octyl) and tetrahalogenoferrate(III) FeX₄⁻ anion (X = Cl and Br), were prepared, and the influence of structural variations such as changing the alkyl chain (R) length in the cation and substituting the halides (X) in the anion on their thermal behavior, infrared and ultraviolet-visible absorption spectra, density, viscosity, ionic conductivity and magnetic susceptibility was characterized. The elongation of the alkyl chain leads to a pronounced reduction of fluidity and ionic conductivity, and the replacement of chlorides with bromides likewise leads to a similar tendency toward ionic diffusion. It would seem that the ion association is less pronounced in the FeBr₄ salts, which can be rationalized on the basis of the more significant nephelauxetic effect together with the lower molar concentration in the FeBr₄ salts.