The carbonisation of biomass to hydrochar has the potential to become an environmentally sound conversion process for the production of a wide variety of products. Besides the use for the production of hydrochars as energy carriers, there are discussions about using hydrochars for environmental, catalytic, electronic and agricultural applications. Since hydrothermal carbonisation of biomass is highly complex and not yet fully understood, this study aims to elucidate the reactions occurring during the process at specific process parameters. Therefore carbonaceous materials were synthesised with a 1.5 litre reactor at 180C and autogenic pressure using different feed materials such as pistachio shells, bamboo sticks, fir needles and biogas slurry. To gain a detailed insight into the chemical and structural properties, carbonaceous hydrochar materials were characterized by SEM imaging and EDX analysis. Structurally, carbon-rich spheres were detected on the surface of the hydrochars; size and shape of those carbon spheres strongly depend on the properties of the feed materials. The formation of these m-sized spheres through hydrothermal carbonisation of biomass is the consequence of dehydration, condensation, or polymerization and aromatization reactions. Chemically, the atomic O/C profiles of cross sections obtained from different hydrochars were determined. Then, the Thiele module is used as a model for the observed O/C profiles, which describes the relation of reactivity to diffusivity of a reactant into a pore. It is shown, that this model fits to the EDX data and therefore it was used to calculate the Thiele Modules, which range from 1.61 0.027 to 1.19 0.019. This should proof, that hydrothermal reactions are hindered by diffusion and thereby reducing its reaction rate. In order to further verify this method, an analysis of different particle sizes is proposed.