The present work concentrates on the thermoanalytic characterization of hydride and phenyl groups chemisorbed at the surface of Aerosil 300. Reaction enthalpies for the oxidation of the hydride and phenyl groups are computed from DTA measurements in DSC mode. Linear relations are received between enthalpy and surface concentrations of the species. The molar reaction enthalpy for the hydride groups is classified into elementary steps in a cyclic process and is compared with theoretically expected molar reaction enthalpies for the oxidation of the possible hydride species at the surface. IR-spectroscopic investigations to the silane absorption band illustrate the existence of different hydride species and support the thermodynamic statements. Thermal stability of the phenyl groups is investigated by TG-MS measurements. Pyrolysis of the groups runs in several steps. Investigations to the isotope exchange lead to conceptions across possible dismantling ways.