This work describes the preparation of polypyrrole and EPDM rubber blends, PPy/EPDM, by the sorption of pyrrole (vapor phase) in an EPDM matrix containing CuCl2. We investigated the effect of the oxidant particle-size on the sorption and polymerization equilibrium, electrical conductivity, and mechanical properties of the blends. Independently of the CuCl2 concentration and polymerization time, the polypyrrole weight fraction in the blend, X(PPy), increases when the oxidant particle-size is changed from 150 250 mum to smaller than 106 mum. For blends containing 50 phr of CuCl2, obtained following 72 h of exposure to pyrrole, an increase in the Young’s Modulus (from 2.2 +/- 0.2 to 3.9 +/- 0.6 MPa) and an increase in the electrical conductivity (from 10(-9) to 10(-7) S cm-1) was observed when the oxidant particle-size was decreased. Infrared spectroscopy, thermogravimetric analysis, scanning differential calorimetry, and scanning electron microscopy were used in sample characterization.