The structure of two biochars and of their activated carbons was investigated by Electron Paramagnetic Resonance, Solid State Nuclear Magnetic Resonance, and Fourier Transform Infrared spectroscopies, together with X-ray diffraction and nitrogen adsorption/desorption isotherm measurements. The biochars were obtained from wood and Posidonia Oceanica by slow pyrolysis up to 600 °C, whereas the activated carbons were prepared from the biochars by impregnation with KOH, heating up to 800 °C. Two different KOH:biochar mass ratios were tested in the case of Posidonia, namely 4:1 and 2:1, while only the 4:1 ratio was used for wood. When the larger ratio was used, activation significantly increased the microporosity of the starting biochar, also creating bottle-neck pores not accessible to water molecules, and induced the formation of larger condensed aromatic networks arranged in interconnected conducting domains. In the case of Posidonia, activation using the 2:1 ratio mainly created mesopores and induced an increase in organic radical content by almost four orders of magnitude. This huge increase was related to the presence of minerals in the starting biochar.