||In France, the deep-seated nuclear waste repository consists of a natural barrier located at 500m depth (in an argillaceous rock named argillite), associated to artificial barriers, including plugs of swelling clay (bentonite) for tunnel sealing purposes. This paper focuses on gas breakthrough tests through bentonite, argillite and the bentonite-argillite interface. The main contribution of our study is to provide insights into the swelling kinetics, gas breakthrough pressure of these materials. During the tests, bentonite/sand plugs and bentonite-argillite plugs are used to perform swelling tests and gas breakthrough tests. Experimental results show that continuous gas passage through fully-saturated bentonite/sand is not obtained until 10.5MPa gas pressure. When a bentonite/sand plug swells inside a smooth metal tube, gas passage occurs at 7-8MPa, which is similar to the effective swelling pressure of bentonite, as measured by independent swelling experiments (Liu et al, 2012). For initially macro-cracked argillite, saturated with water until sealing (water permeability on the order of 10-20 – 10-21 m2), gas breakthrough may occur at as low a pressure as 0.2MPa and up to several MPa (depending on argillite orientation); it has been recorded up to 6MPa for undisturbed argillite (Davy et al, 2012). Finally, also after full water saturation, continuous gas breakthrough through the bentonite-argillite interface is detected at between 7~8MPa gas pressure. It is interpreted that the argillite rock or the bentonite-argillite interface are both preferential pathways for gas migration when all materials become fully saturated.