The Boom Clay Formation has been selected as a model for studying the long term disposal of high activity nuclear waste in clay. During the resulting storage, the immature kerogen in the clay would be subjected to thermal stress and some of the products which could be generated might affect the efficiency of the geological barrier. The kerogen isolated from a representative clay sample, collected in the underground laboratory of the Nuclear Energy Research Centre at Mol (Belgium), was subjected to short heating at 300 °C to examine the thermolabile fraction released. The fraction contains a wide variety of components including hydrocarbons, ketones, alkanoic acids, phenols, furans and pyrroles. Information about the origin of these compounds was obtained through comparison with the bitumen and with the previously examined pyrolysis products generated from the kerogen at higher temperature (400 °C). The thermolabile fraction appears to correspond to “free” compounds trapped in the macromolecular structure of the kerogen and “labile” moieties linked by covalent bonds with a relatively low thermal stability. The study affords additional information on the sources and composition of the kerogen. Furthermore, the thermolabile components in the Boom Clay kerogen might affect the effectiveness of the clay barrier when subjected to thermal stress, especially through the release of oxygen-containing products, like carboxylic acids and phenols, that might complex released radioactive elements.