Waste newsprint fibers were used as reinforcing fillers in composite materials based on a styrene-butadiene rubber matrix. Surface modification of the fibers was carried out by polystyrene emulsion treatment, and copolymerization with the rubber matrix was done by a radiation technique. Characterization of the modified surfaces and the adhesion between the treated newsprint paper fibers and the rubber matrix involved a variety of techniques, including X-ray diffraction, FTIR spectroscopy and differential scanning calorimetry, which revealed that the surface modification did occur and that it affected adhesion between the treated fibers and the rubber matrix upon irradiation. Thermogravimetric analysis indicated that the thermal stability of the fibers was enhanced by the treatment and that the thermal stability of irradiated composites was increased by the addition of treated newsprint fiber. © 2012 Society of Plastics Engineers.