The development of accurate kinetic models for hydrothermal carbonization (HTC) faces major difficulties. This is related to the fact that the formation of the solid hydrochar can be regarded as a formation of a second phase from a homogenous solution. The reaction mechanism is not fully understood. From the current state of art, it is obvious that the reaction mechanism is some sort of polymerization reaction, thus, having reaction order higher than one. In order to gain more knowledge about the HTC reaction, fructose is used as a model substance, as it can be regarded as a key intermediate during the hydrothermal processing of carbohydrates. The results indicate that the reaction order is a highly sensitive parameter varying with temperature, reaction time and initial concentration. It can be concluded that the model itself is a strong simplification of the real reaction mechanism, thus more research is necessary to complete the actual knowledge.