CoFe2O4 magnetic nanoparticles were prepared by in situ precipitation and oxidation of Co2+ and Fe2+ within a sulfonated polystyrene resin. The nanometric particles were characterized by X-ray diffraction. A ferrofluid was prepared from the CoFe2O4 mineralized polymer resin and water. Poly(vinyl alcohol) (PVA)-based nanocomposite materials were obtained by mixing different amounts of ferrofluid (compositions ranging within 0-51 wt % of mineralized resin),with an aqueous solution of the polymer. The PVA composite materials were characterized by TGA, DSC, and stress-strain testing. The thermal and mechanical properties of PVA change with filler content, exhibiting an initial increase in these properties due to polymer-filler interactions. After a maximum value, at about 15 wt % of mineralized resin, the mechanical properties decrease probably due to particle aggregation which causes phase separation. The results obtained show that the nanoparticles are dispersed in the amorphous regions of the polymer, the crystalline zones remaining unaltered up to compositions as high as 30 wt %.