Magnetic nanocomposites were prepared by an in situ oxidative polymerization method to encapsulate different loadings of iron oxide nanoparticles (MNP) by a conductive polymer, polypyrrole (PPy), and were blended into an epoxy resin matrix. The morphology, DC conductivity, magnetic, and electromagnetic interference (EMI) shielding behaviors of samples dispersed in the resin were characterized, the latter by use of a vector network analyzer in a frequency range of 0.1-18 GHz. Nanocomposites based on the use of MNP/PPy composite nanoparticles in which the magnetic and conducting phases coexist in intimate contact showed a marked increase in the absorption 10.10 dB at the maximum frequency limit (17-18 GHz) of the instrument, in comparison with the absorption bands for PPy particles only (7.5 dB) or MNP only (2.6 dB) or physical blends of MNP and PPy particles (3.6 dB) in the resin. The mechanism of this enhancement is discussed based on electromagnetic theory. POLYM. ENG. SCI., 51:247-253, 2011. (C) 2010 Society of Plastics Engineers