In this study the microwave-assisted functionalization of high purity carbon nanotubes (BayTubes (R)) with well-known oxidation agents such as 5 and 10 M nitric acid, 10% hydrogen peroxide solution, a mixture of both, or 0.2 M potassium permanganate, is investigated and compared to CNTs treated by conventional oil bath heating. The obtained CNT materials are characterized with a variety of bulk and surface characterization techniques (XPS, Raman, TGA, TEM) and the obtained results could be correlated successfully with the severity of the treatment and the electrochemical properties of the CNTs. Depending on the oxidation agent oxygen containing functional groups are introduced into the CNTs where the chemical constitution of the functional groups in a hydrogen peroxide treated sample is significantly different from those by all other treatments. As a result the electrochemical behavior is strongly influenced which is clearly demonstrated by the absence of the quinone-type redox couple in this sample, which is found in all other samples. Finally, the time dependence of a nitric acid treatment is investigated and the obtained results allow setting up tailored procedures for functionalization of high purity carbon nanotubes resulting in materials with a controlled amount of oxygen containing functional groups that can be used for a variety of further synthesis steps. (C) 2015 Elsevier Ltd. All rights reserved.