Nanometric ferrihydrite, maghemite and magnetite particles formed within an organic-inorganic hybrid matrix were obtained by the sol-gel process. In contrast to precipitation techniques, sol-gel process appears as suitable way to achieve size-controlled nanoscopic magnetic particles anchored in a hybrid structure. The hybrid matrix here reported, named di-ureasil, is composed of poly(oxyethylene) chains grafted to siloxane groups by means of urea cross-linkages. The formation of ferrihydrite particles was achieved incorporating iron nitrate during the sol-gel process, at low pH. The formation of maghemite takes place after the incorporation of a mixture of Fe3+ and Fe2+ ions and treatment with an ammonia solution, after the sol-gel process. Magnetite nanoparticles are formed after the incorporation of Fe 21 ions and treatment with ammonia at 80 degrees C. The AC magnetic susceptibility shows thermal irreversibility with a blocking temperature T-B approximate to 13K and approximate to 25K depending on frequency for the ferrihydrite and maghemite particles, respectively. The magnetite nanoparticles are blocked at room temperature. Above the irreversibility the magnetization of ferrihydrite and maghemite follows a Langevin function modified with a linear term, as found in antiferromagnetic and ferrimagnetic particles.