Superparamagnetic properties of self-aggregated nickel nanoparticles in a perfluorinated sulfo-cation membrane (MF-4SK) prepared by ion-exchange and recovery methods were investigated by transmission electron microscopy (TEM) and a superconducting quantum interference device magnetometer. The TEM image shows that the Ni nanoparticles are well-dispersed in the MF-4SK (5 x 10(19) Ni atoms per I g of MF-4SK) with a narrow size distribution of 1.5-3 nm in radius. The Ni nanoparticles in the MF-4SK exhibit superparamagnetic properties above the blocking temperature. The particle radii deduced from the analysis of the magnetic susceptibilities (2.7 nm) and magnetization curves (1.8 nm) are reasonably consistent with the TEM observation. This experimental evidence suggests that the Ni nanoparticles in the polymer film should be considered to have a single magnetic domain. The results are discussed on the basis of the superparamagnetic behavior of magnetic nanoparticles. (C) 2004 Elsevier B.V All rights reserved.