Structure and magnetic properties of polymer matrix nanocomposites, processed by pyrolysis of the Fe(III)Co(II) acrylate complex, were investigated. It was shown that thermal transformation of the complex studied consisted of three macrostages: dehydration, solid state polymerisation and decarboxylation of a metallopolymer form. The main products of the decomposition were nanoparticles stabilised by polymeric matrix. The crystalline phases, which were found in the fully processed material, were Fe3O4, CoFe2O4 and CoO. The mean crystallite size was 10nm. In the intermediate stages of the thermolysis iron was present in the forms of FeIII (trivalent low – spin iron), Fe2+(divalent high – spin iron) and Fe3O4. The hysteresis loops measured at temperatures below 200K were opened and shifted towards negative field. The coercivity and remanence showed room temperature values of 0.18T and 15.5mT, respectively.