The invention provides a process for preparing polymer beads incorporating magnetic iron oxide particles, which process comprises producing a dispersion having a continuous aqueous phase and a dispersed organic phase, the organic phase comprising one or more polymerisable monomers, magnetic iron oxide particles and an organophosphorus dispersing agent for dispersing the magnetic iron oxide particles in the organic phase, and polymerising the one or more polymerisable monomers to form the polymer beads incorporating the magnetic iron oxide particles; complexing and ion exchange resins prepared by this process; methods for separating transition metal ions and other ions from an aqueous solution using the complexing and ion exchange resins; and polymer beads comprising a polymeric matrix having magnetic iron oxide particles and organophosphorus dispersing agent dispersed substantially uniformly therein.