The invention relates to a process for preparing an agglomerate-free suspension of nanoscalar iron oxide particles coated in a stable manner. Said process comprises the following stages in the given order: (1) preparation of an aqueous suspension of nanoscalar iron oxide particles which are partially or completely in the form of agglomerates; (2) addition (i) of a trialoxysilane which has a hydrocarbon group bonded directly to Si, to which group at least one amino group, carboxyl group, epoxy group, mercapto group, cyano group, hydroxy group and/or (meth)acryl group is bonded and (ii) addition of a polar organic solvent which can mixed with water and has a boiling point at least 10 DEG C higher than the boiling point of water; (3) treatment of the resulting suspension with ultrasound until at least 70 % of the particles present are in the range of +/- 20 % of the average particle diameter; (4) removal by distillation of the water by the action of ultrasound; and (5) separation of the non-disintegrated agglomerates.