AbstractMagnesium materials are of increasing interest in the development of biodegradable implants as they exhibit properties that make them promising candidates. However, formation of gas cavities after implantation of magnesium alloys is widely reported in literature. The gas composition and the concentration of in these bubbles are not clearly known as only a few studies using techniques not specific for were done about 60 years ago. Currently, many researchers assume that these cavities contain primarily hydrogen because it is a product of magnesium dissolution in aqueous media. In order to clearly answer this question, we implanted rare earth containing magnesium alloy disks in mice and determined the concentration of hydrogen gas for up to ten days using an amperometric hydrogen sensor and mass spectrometric measurements. We were able to directly monitor the hydrogen concentration over a period of ten days and to show that the gas cavities only contained a low concentration of hydrogen gas, even shortly after the formation of the cavities. This means that hydrogen must be exchanged very quickly after implantation. To confirm these results, hydrogen gas was directly injected subcutaneously and most of the hydrogen gas was found to exchange within the first hour after injection. Overall, our results disprove the common misbelief that these cavities mainly contain hydrogen and show how quickly this gas is exchanged with the surrounding tissue.