This study proposes new approaches for measuring the gas permeability and the accessible porosity of porous media. Two techniques are used: the usual permeameter, of the Cembureau type (measurement under pressure), and a new technique named a "double-cell" permeameter, based on a vacuum technique. Theoretical and experimental results point out that the apparent permeability measured in vacuum is proportional to the permeability measured under pressure. For a given pressure, the theoretical expression of the coefficients of proportionality leads to a quasi-constant value for a very large range of concrete permeability. A new equation is also proposed to evaluate the accessible porosity from the Time to Reach Steady State (TRSS) recorded during permeability tests. Concordance between the porosity accessible to gas obtained in this way and the porosity measured by the usual technique of hydrostatic weighing is discussed.