10 The transient method of the mass flow rate measurements through a microp-orous media is developed and analyzed. This method is based on the constant volume technique and the exponential fit of the pressure evolution in each tank which allows calculating the permeability directly. The pressure relaxation time, a single fitting parameter, is introduced and its behaviors are analyzed in a large pressure range. By measuring the pressure relaxation time for one gas, the per-meability of a microporous sample can be derived for the other gases. With the actual experimental setup, we measured the mass flow rate through the micro-porous media in the range 5 · 10 −7 − 5 · 10 −12 [kg s −1 ] and the permeability in the range 10 −14 − 10 −11 [m 2 ].