[en] Measurements of the critical gas pressure of intact samples of Kimmeridge and London Clay along with measurements on London Clay of gas diffusion in solution were made in the laboratory. The critical gas pressure, the pressure required to overcome the capillary pressure in the pores of the samples, ranged from 3.5 to 6.2MPa. The intrinsic diffusion coefficients obtained for migration at low pressure in solution were of the order of 10^-11 m²s^-1. The samples were confined in test cells which simulated burial depths in excess of 200m and used nitrogen gas saturated with water. (author)