This paper presents a small-angle X-ray scattering (SAXS) characterization of the structural organization of deposits of Fe-montmorillonite dispersions formed during frontal ultrafiltration in a magnetic field. The mineral colloidal dispersions are made up of platelike montmorillonite clay particles with average thickness and diameter of about 2 and 500 nm, respectively. A newly developed X-ray-compatible frontal ultrafiltration cell allowed simultaneously applying a transmembrane pressure (5 × 104 Pa) and a magnetic field. The Fe-montmorillonite dispersions obtained by cation exchange reaction displayed regular ordering of the particles with an anisotropic arrangement when subjected to a uniform magnetic field of strengths ranging up to 1.4 T. The degree of anisotropy and the particle concentration profiles as a function of the distance from the filter membrane were deduced from the measured SAXS intensity. For the same volume of filtered permeate (0.26 mL), the deposit formed in a magnetic field of 1 T exhibits an anisotropic arrangement of the platelike particles with their faces aligned parallel to the membrane. In the absence of a magnetic field, the deposits are composed of randomly oriented particles. The application of an external magnetic field produced uniaxially oriented deposits with a higher concentration of particles, and resulted in a higher permeation flow than in the absence of the field.