Carbon nanotubes (CNT) combine high electrical conductivity with good chemical and mechanical properties, making them interesting for many applications, such as reinforcement in polymer composites. In order to better adapt the tubes to specific applications, they can be modified in various ways. Our research focuses on the functionalization of single-walled and multi-walled carbon nanotubes with different oxidation agents. By adding carboxylic groups to the surface defects of CNT, the interactions between the tube surface and the epoxy resin can be improved. This results in a stronger CNT-matrix bonding and thus improves the mechanical properties of the composite material. The CNT are synthesized by chemical vapor deposition and afterwards functionalized in different steps. The first step is the purification of the tubes to remove catalytic particles and amorphous carbon. This is followed by oxidative functionalization by using different oxidative acids like nitric acid, sulfuric acid and mixtures of the acids to include carboxylic groups at the tube surface as the second step. The reactions will be carried out by reflux and sonication with varying the parameters like reaction time and temperature, in order to investigate the influence of the reaction parameters on the degree of functionalization and its effects on the tube surface structure. The influence of the functionalization method on the dispersive behavior of the functionalized CNT in epoxy is investigated as well as mechanical properties of the resulting nanocomposites by fracture toughness and tensile tests. The characterization of the acid treated tubes is carried out using different analysis methods. The effect of functionalization on the CNT-matrix bonding is investigated by fractography of the nanocomposite fracture surfaces in SEM. With this approach, the potential of CNT functionalization for improving physical properties of nanocomposites and for use in different applications is analyzed.