Magnetic cation-exchange resins usually consist of small polymeric particles with a magnetic material dispersed within their structure. Because the mass-transfer kinetics through these solid particles depends on their size, very small diameter particles are desirable. Nevertheless, faster kinetics can also be obtained by using particles consisting of spherical magnetic cores covered with a thin layer of polymeric adsorbent material. In the present work, we describe the synthesis of a novel type of ion-exchange resin, in which the magnetic core was a single spherical stainless steel particle and the covering layer was poly(styrene-co-divinylbenzene). To check their ion-exchange characteristics, the equilibrium adsorption of the amino acids methionine, phenylalanine, and tryptophan on magnetic and nonmagnetic resins was investigated. The experimental results are explained in terms of a model that considers that the surface of the ion exchangers is heterogeneous, with two different types of adsorption sites with different selectivity coefficients. A higher fraction of sites with lower selectivity was found in the magnetic resins, resulting in adsorption isotherms slightly less rectangular than the isotherms obtained for the nonmagnetic resins.