Abstract: A process for preparing hexaferrite particles of the type M'Fe.sub.12 O.sub.19 wherein M' represents principally an alkaline earth metal involves preparing a particulate mixture of a ferrous salt of an organic acid and a salt of an organic acid with at least one metal M', optionally with at least one dopant agent, the particles of the mixture having a size not greater than about 0.5 .mu.m and the said mixture containing a stoichiometric excess of the salt of metal M'. This mixture is heated up to the decomposition of the organic salts and the resulting product is submitted to a thermal treatment at a temperature at least equal to 700.degree. C. but not greater than 1000.degree. C. and is maintained at this temperature for a period of time sufficient to form hexaferrite. After cooling the resulting particles are washed with an aqueous solution of an acid capable of dissolving formed monoferrites of the type M'Fe.sub.2 O.sub.4 to remove them.