The bitumen used for the embedding of nuclear wastes has been pyrolysed in confined and hydrous systems, in order to evaluate while an elevation of the temperature in the deep geological repository could induce chemical transformations of the bitumen matrix, which could influence its confining properties. The pyrolysates have been extracted in chloroform and the extract has been separated in aliphatic aid aromatic hydrocarbons, resins and asphaltenes. These different fractions have been analyzed by GC-MS and mu -IRTF. Few modifications of the bitumen structure are observed up to 300 degreesC. Above this temperature, the pyrolysis of the bitumen generates light hydrocarbons and ai aromatic and condensed insoluble residue. The use of the time-temperature index allows to estimate the temperatures which have to be reached in the repository in order to observe the first transformations of the bitumen.