Methods of preparation and the properties of polymer composites with superparamagnetic nanoparticles are reviewed and discussed. Multicomponent polymer systems with nanoparticles of metal oxides can be tailored to exhibit, besides good magnetic properties and transparency, also good adhesion (polymers and copolymers of various types), film forming and processing properties. An interesting feature of polymer composites with nanosized superparamagnetic particles is that, in all cases reported, the measured magnetization considerably deviates from the values calculated or measured for bulk iron oxides (e.g. single crystals). No definite explanation of this discrepancy exists. The studies on a selected system: polyaniline/iron oxide particles help to resolve this question because interactions: between the matrix and iron oxide nanoparticles can be studied under different conditions and at different stages of structure formation. (C) 1998 Elsevier Science S.A. All rights reserved.