Bestaendige gamma-Eisen(III)-oxyd enthaltende Sole

The invention comprises magnetic sols of g Fe2O3 which are stable for at least two months and which show no increase in size as determined by electron microscopy. The sols may contain up to 38% by wt. g Fe2O3 of particle size 3-12 mm and the continuous phase may be water or a non-polar organic liquid, e.g. toluene. Stable aquasols are prepared by (a) adding, preferably in an inert atmosphere, an aqueous solution of ferric and ferrous salts of a monovalent anion (e.g. Cl-,NO3-) in which the molar ratio Fe (III): Fe (II) is 3:1 to 1:1 and the total concentration of Fe is 0.5-3M, to an agitated slurry of an anion exchange resin of the strong base type in the OH form present in an excess of 3-200% by volume of the anions to be removed, such that the pH is maintained at 7.5 to 9.5, (b) stabilizing the sol by reducing the pH to 1-3 rapidly by adding a mineral halide and/or an acid. The sol may be dialysed at 10-30 DEG C. to form a gel which may be separated form excess water and redisposed in water at pH 1-3 to give an aquasol of increased concentration or contacted with a lyophilic transfer agent (e.g. ammonium oleate) in an organic liquid to form an organosol. Solid hydrated g Fe2O3 may be prepared from the aquasol by (a) evaporating the sol in air, (b) adding alkali until the pH is 3.8 or (c) adding MgSO4 and drying the precipitate of Fe2O3 in each case at up to 200 DEG C.