Reports on the use of bleached Eucalyptus cellulose as a reinforcing agent for polypropylene and on the use of lignin, from Eucalyptus and other vegetal species, as a stabilizer for different polymers can be found in the scientific literature. This work focuses on polypropylene composites using cellulose fibers with different bleaching levels, i.e. containing controlled lignin contents. We compared the properties and stability of composites with bleached and with semi-bleached fibers. These were prepared by twin-screw extrusion and injection molding and characterized by their tensile and flexural mechanical properties, thermogravimetry, oxidation induction time, reflectance infrared spectrophotometry and scanning electron microscopy. The kinetic evaluation of the accelerated and environmental aging of the injection molded test samples was followed by mechanical properties variation and reflectance infrared spectrophotometry. An increase in the reinforcing effect of the fibers and on the stability of the composites was observed for composites containing semi-bleached Eucalyptus fibers, with different lignin contents, compared to composites with the bleached cellulose fibers. The lignin present in these semi-bleached fibers positively affected the mechanical properties of the composites and the hindered phenols in the lignin structure acted as a primary anti-oxidant, indicating their advantageous use in comparison to the bleached fibers. Scanning electron micrographs showed that the excellent bulk fiber/matrix adhesion was not affected by accelerated or environmental aging.