Currently is a tendency to obtain activated carbons (ACs) from cheap and renewable raw materials by methods requiring low energy consumption. A perspective research line in this context is the use of microwaves in the process of ACs preparation. The employment of microwaves is finding a widening application in various fields because of the many advantages of microwave heating in comparison with traditional methods, such as fast heating, lack of inertia in heat transfer and activation time reduction, which in turn lead to diminished consumption of energy and activation agents. The oxidation of ACs aims at the formation of various functional groups on their surface for a stronger interaction with nanoparticles or biological active compounds. Acid and base functional groups are also formed during the production of ACs, depending on the activation methods employed. However, the amount of these groups is very small, mainly because of their instability at the elevated temperatures at which the activation process takes place. The advantages provided by gaseous ozone oxidation cannot be achieved by using other oxidizing agents: the process takes place at room temperature, the structure of the activated carbon is not contaminated with heteroatoms as a result of oxidation, functional groups of a certain type are formed and the surface of the AC is sterilized during the process. These features make it possible to obtain ACs suitable for use in medicine. ACs from walnut shells were obtained by microwave treatment. To increase the functionality of the adsorbent, the ACs were oxidized with ozone, resulting in an adsorbent containing aliphatic and aromatic carboxylic groups. There was a considerable decrease in the specific surface of the activated carbon after the oxidation process. Nitrogen adsorption was used to determine the structural parameters of the ACs. A simultaneous thermal analysis was used to study the thermal behaviour of intact and oxidized activated carbons. In order to estimate the thermal stability of the oxidation-based functional groups and to observe the reversibility of the chemosorption process, a thermal analysis of the intact and oxidized samples was performed. The correlation of the results obtained from the thermal analysis and nitrogen adsorption indicated that the ozonization process led to the formation of functional groups of predominantly acidic nature on the surface of activated carbon; the oxidation took place homogeneously throughout the surface, leading to the blocking of pores and a considerable reduction of the specific surface; ozone chemosorption was partially reversible after desorption at elevated temperatures leading to incomplete restoration of the specific surface, while the morphological structure of the adsorbent underwent insignificant changes.