Alginate-based magnetic gels were prepared in a sequence of procedures for in situ synthesis of iron oxides: crosslinking of alginate in ferrous salt aqueous solution, immersion of the resulting gels into alkaline-earth metallic hydroxide solution, and treatment of the partially cation-exchanged gels with hydrogen peroxide. Magnetometry measurements revealed that the Iyophilized gels showed superparamagnetism or ferromagnetism at room temperature, depending on the sort of the metallic cation employed in the alkali treatment. An interpenetrating network (IPN) type of alginate/poly(vinyl alcohol) (Alg/PVA) gels containing iron oxides was also designed and fabricated from mixed polymer solutions by a modified preparation method; the gelation and alkali treatment were carried out with the aid of a metallic borate. Viscoelasticity of the magnetic lPN composites in the gelatinous state was controllable by changing the mixing ratio of the compatible carbohydrate/viny polymer pair. Magnetization versus magnetic field curves were constructed at different temperatures for the IPNs freeze-dried. Their magnetic character was variable according to alterations of the Alg/PVA composition, alkaline reagent adopted, and measurement temperature. (C) 2004 Elsevier Ltd. All rights reserved.