Poly ether ether ketone (PEEK)/Fe composites were prepared by a twin-screw extruder in which 30 and 35 %wt. of iron microparticles were dispersed into medium viscosity PEEK polymer. Samples for further characterisations were manufactured by an injection moulding process. The PEEK crystallinity decreased at these loadings of iron particles due to the reduction of chains movement in the polymer. The temperatures of chain-scission and thermo-oxidation as two decomposition stages of the composites showed a subtle change compared to the pure PEEK polymer, which suggests an influence of the additive material.