Method for filling at least one buffer container (1, 2, 3) of a pressurised hydrogen gas filling station, the filling station comprising at least one buffer container (1, 2, 3) and one fluid circuit (11, 12, 13, 4, 5, 6) connected to said at least one buffer container (1, 2, 3), the circuit (11, 12, 13, 4, 5, 6) of the filling station comprising a first end connected to at least one source of hydrogen gas (14, 15) to provide for the filling of the at least one buffer container (1, 2, 3) with gas coming from the source (14, 15), the circuit (11, 12, 13, 4, 5, 6) comprising a second end provided with a transfer conduit intended to be removably connected to a tank, to fill said tank using at least one buffer container (1, 2, 3) with hydrogen gas that has a concentration of at least one predefined impurity less than a predefined concentration threshold, the method being characterised in that it comprises a step of determining (9, 10) the current concentration of at least one impurity in the hydrogen in the buffer container (1, 2, 3) during the filling of same, a step of comparing said current concentration of the impurity relative to a predefined threshold concentration and, when the current concentration of the at least one impurity reaches said threshold concentration, stopping the filling of said buffer container (1, 2, 3), the step of determining (9, 10) the current concentration of at least one impurity in the container comprising at least one from among: - a measurement of the concentration of the impurity directly in the buffer container via sensor(s) and analyser(s), - a measurement by sensor(s) of the concentration of the impurity at the level of the at least one source (4, 5) of hydrogen and/or at the level of the fluid circuit of the filling station and a determination of the quantity of gas transferred in real time into the buffer container (1, 2, 3) during the filling of the latter said current concentration being determined by calculation using said concentration of the impurity and of the quantity of gas introduced into the buffer container, the determination of the quantity of gas transferred in real time into the container (1, 2, 3) during the filling thereof comprises at least one from among: a measurement via a flow sensor in the circuit (11, 12, 13, 4, 5, 6), a measurement of the variation in pressure in the buffer container (1, 2, 3) during the filling of the latter. - a measurement of the concentration of the impurity directly in the buffer container via sensor(s) and analyser(s), - a measurement by sensor(s) of the concentration of the impurity at the level of the at least one source (4, 5) of hydrogen and/or at the level of the fluid circuit of the filling station and a determination of the quantity of gas transferred in real time into the buffer container (1, 2, 3) during the filling of the latter said current concentration being determined by calculation using said concentration of the impurity and of the quantity of gas introduced into the buffer container, the determination of the quantity of gas transferred in real time into the container (1, 2, 3) during the filling thereof comprises at least one from among: a measurement via a flow sensor in the circuit (11, 12, 13, 4, 5, 6), a measurement of the variation in pressure in the buffer container (1, 2, 3) during the filling of the latter.