Laser beam welding with partial gas shielding using local gas flows has been shown to be very effective in reducing spatter, especially when welding high-alloy steels at relevant processing speeds (≥ 8 m/min). Although several investigations have clarified the mechanical, thermal and chemical metallurgical effects, the penetration behavior of the locally supplied gas is still unknown. Therefore, this paper investigates the depth of penetration of locally supplied argon during keyhole mode LBW of high-alloy steel AISI 304 by means of in-situ gas extraction and analysis. As a first step, an in-situ measurement of the inner keyhole pressure provided fundamental information about the effect of the gas-induced dynamic pressure on the inner keyhole pressure. This information was supported by high-speed synchrotron X-ray imaging, which allowed the gas-induced pressure to be correlated with significant changes in keyhole geometry. An analysis of the chemical composition of the metal vapor and gas mixture trapped in the keyhole was carried out to reveal a possible penetration of the locally supplied gas into the keyhole. For this purpose, the gas mixture was extracted at several different positions within the keyhole during the welding experiments and analyzed by mass spectrometry. Overall, the gas analysis showed no gas penetration and supports the fact that the gas flow only impedes the outflowing metal vapor and could lead to a metal vapor backflooding within the keyhole.