Abstract Reused tyres whose surfaces were treated with various chemical acids, such as H 2SO 4, HNO 3 and HClO 4, were used as reinforcement material in HDPE-reused tyre composites. Their mechanical properties (e.g. tensile strength, Young’s modulus, toughness and elongation at break) were studied to evaluate how surface treatments may improve compatibility between the two components. The effect of chemical modifications on the surface of reused tyres was monitored by FTIR, the determination of mechanical properties and SEM. The importance of rubber treatment can be assessed by comparing the results of treated and untreated composites with those for neat HDPE. Reused tyre rubber, added to the HDPE in small quantities, acts as a filler, improving the stiffness and providing a more brittle behaviour. Nevertheless, a rubber content above 10%, using either untreated rubber or rubber treated with HClO 4, gives lower values of Young’s modulus than neat HDPE. Elongation and toughness values are also lower. Treatment with H 2SO 4 and HNO 3 improves the ability of rubber to interact with the HDPE, increasing the material’s stiffness. The treatment with H 2SO 4 was the most effective, whereas HClO 4 did not improve the material’s properties.