||A complete poromechanical study has been carried out on COx argillite which is currently foreseen by Andra (in France) for the storage of high-level and long-life nuclear waste. The main goal of this study was to measure Biot's coefficient for samples cured at different relative humidity. To overcome the difficulties which occur with water as the fluid to control the pore pressure of argillite, a new technique has been used with gas injection (argon) was intended to measure the coefficient in this contribution. The results unambiguously showed that Biot's tensor components were isotropic, and equal to 1 for partially saturated samples. They revealed to be independent on the level of confining pressure or pore pressure. Unexpected results were however obtained for the dry materials for which the components, larger than 1, were measured, depending on the level of pore pressure. It was therefore suspected that an additional swelling phenomenon occurred with argon due to its adsorption in the micropores of argillite. Complementary experiments were thus carried out with helium leading to the coefficients virtually equal to 1 for the dry material, as helium does not adsorb at ambient temperature. Hence, it evidenced that there was a swelling deformation due to argon adsorption leading to apparent Biot's coefficients greater than 1.