The paper addresses the issue of the description of the coupled thermo-hydro-mechanical behaviour of argillaceous rocks. A constitutive law, developed for this type of materials, is firstly described. It is based on an elastoplastic approach combined with damage concepts. Theoretical formulation and constitutive law are then used to analyze and interpret the observations gathered during the performance of an in situ heating test carried out in an underground laboratory. As a result of the analysis, a better understanding of the relationship between the various interacting phenomena is achieved.