Mössbauer and magnetic characterization of polymer-dispersed γ-Fe2O3 nanoparticles treated under different chemical processes are reported in this work. X-ray powder diffraction analysis provides a mean particle size of D ~ 8.0 nm. Whereas Mössbauer spectroscopy data suggest the presence of only Fe3+ ions, magnetization measurements indicate the occurrence of a freezing phenomenon in agreement with the thermal evolution of Mössbauer spectra. A core-shell model was used to determine a magnetically disordered layer (shell) of d ~ 1.0 nm covering a region of collinear magnetic moments (core). The chemical treatments with H2O2 and Na2S2O8 modify notoriously the magnetic response of the polymer-dispersed nanoparticles. © Springer Science + Business Media B.V. 2009.