In recent years, the consumption of plastics has been increasing and as consequence, the waste generated has also increased. Rubber tire (RT) waste is another residue which causes significant problems to society. In view of the considerable amounts of RT waste generated, this study aimed to evaluate the influence of the incorporation of RT particles into post-consumer thermoplastic matrices such as polypropylene (PP), high impact polystyrene (HIPS) and PP/HIPS blends and the modification of the physical, morphological, rheological and mechanical properties. The particle sizes of the RT used were <500 and 500-1000 μm. The RT content was 10% w/w and the weight ratio for the PP/HIPS blend was 4/1, with processing by injection molding. The results showed that the smaller (500 μm) particle size led to a decrease in the melt flow rate (MFR) of the PP/RT composites (increased viscosity) and an increase for the HIPS/RT composites. On the other hand, the larger particles (1000 μm) led to a decrease in the mechanical performance of the PP/RT and HIPS/RT blends when compared with the neat polymers (PP and HIPS post-consumer). The observed decrease in the mechanical properties of these composites was due to weak filler/matrix interactions, which can be visualized in images by scanning electron microscopy (SEM) of the fracture surface after tensile testing.