Nonstoichiometric iron (III) oxichloride is capable of intercalating a large number of guest molecules with Lewis base character, e.g. dimethylsulfoxide, formamide, hexa-methylphosphortriamide, or alkylamines, while the reactions of stoichiometric FeOCl are restricted to pyridine and its derivatives only. The nonstoichiometric host lattice can be synthesized either by direct reaction form Fe2O3, FeCl3 and Fe or by reduction of the stoichiometric compound. In the latter case the basal spacings of the intercalation complexes depend upon the degree of reduction and the electrolyte concentration. The highly expanded lattice is rapidly hydrolyzed in the presence of water.