During excavation of galleries within the Boom Clay, the physico-chemical conditions are significantly modified as an air-clay interface is created. In order to study the long-term impact of the air-clay contact on the organic matter contained in the Boom Clay, two types of samples were studied: (1) a reference series of clay samples having been in contact with the atmosphere of the HADES gallery for increasing times up to several years and (2) unaltered clay samples submitted to artificial oxidation in a ventilated oven at 80 degrees C. The evolution of geochemical data of the two series was compared using Rock-Eval pyrolysis, GC-MS and size exclusion chromatography. The evolution of geochemical parameters during air alteration for the two series are very similar. They show progressive oxidation of kerogen accompanied by the release of bitumen enriched in low molecular weight constituents. The results show that (1) air oxidation is a major process in the in situ alteration of the organic matter of Boom Clay within the HADES galleries, (2) laboratory oxidation experiments at 80 degrees C yield similar results as in situ air alteration of Boom Clay.