Fe + ions (40 keV) were implanted into polyethyleneterephthalate (PET) films with fluences of (0.25–1.5) · 10 17 cm À2 . Magnetic prop-erties of the synthesised Fe:PET composites were studied using superconducting quantum interference device (SQUID) technique in tem-perature range of 2–300 K. For range of fluences (0.5–0.75) · 10 17 cm À2 the samples reveal superparamagnetic behaviour at room temperature. At fluences above 0.75 · 10 17 cm À2 the strong increase of magnetisation and transition to ferromagnetic properties are reg-istered. Analysis of the magnetic hysteresis loops suggests an easy plane magnetic anisotropy similar to that found for thin magnetic films. Zero-field-cooled (ZFC) and field-cooled (FC) temperature measurements of magnetisation are found to be in agreement with earlier observed formation of Fe nanoparticles (NPs) in the implanted layers. The growth and agglomeration of the NPs forming the quasi-continuous labyrinth-like structure in the polymer film at the highest implantation fluence of 1.5 · 10 17 cm À2 is an origin for the transition to the ferromagnetic properties.