t: Three commercial formulations of polypropylene (PP) and two of polyethylene (PE) were ã- irradiated with 25 kGy integral dose in air atmosphere and investigated using X-band electron paramagnetic resonance (EPR). The free radicals were identified and their concentrations were related to both formulation and crystallinity of the polymers. The recombination of each species was monitored versus temperature. The concentration decay versus time concentration was monitored and simulated as a mixture of zero or first order mechanism for HDPE and of one for the PP polymers. In order to improve impact and tensile properties of PP based blends containing PE, reactive extrusion has been carried out with radicals obtained by ã-irradiation. The addition of a ã-irradiation step in the processing sequence, between extrusion and injection, can be considered as an efficient way for improving the properties of blends. The improvement of mechanical properties has been discussed in terms of creation of secondary interactions or covalent bonds at the interface of the ã-irradiated polymers, ã-PP and ã-PE. Mechanical, rheological properties and Electron Paramagnetic Resonance (EPR) studies of the extruded blends after ã-irradiation reveal that reactions of ã-PP and ã-PE macroradicals are likely to occur. On one hand, the rheological measurements have highlighted an increase of the complex viscosity of the ã-irradiated blends at low frequencies. On the other hand, the EPR spectrum simulation shows that the radicals are not the same in the blend and in the genuine ã-PP and ã-PE. Besides, the increase of temperature in the sample affects the shape of the EPR spectra, revealing that some macroradicals react one to another, which creates new macroradical structures of rather alkoxyl type. Key words: Polyolefin blends, radicals, reactive extrusion, high shear, ã-irradiation, electron paramagnetic resonance (EPR), simulation, reinforcement, lightening.