As magnesium and many of its alloys are a promising class of degradable implant materials, a thorough understanding of their degradation under physiological conditions is a key challenge in the field of biomaterial science. In order to increase the predictive power of in vitro studies, it is necessary to imitate the in vivo conditions, track the decomposition process and identify the products that form during the degradation pathway. In this in vitro study, slices of pure magnesium were exposed to Hank's Balanced Salt Solution (HBSS), Dulbecco's Modified Eagle Medium (DMEM) and simulated body fluid (SBF), respectively, under cell culture conditions, which included CO2 gassing. The series were repeated with supplements of fetal bovine serum (FBS), added to the respective media. Degradation rates, osmolality and pH were found to vary with the choice of medium and supplementation with proteins. In order to identify the crystalline degradation products, the crusts formed on the specimens were investigated via X-ray diffraction (XRD) measurements. As expected, brucite, Mg(OH)2, was found among the degradation products; interestingly, nesquehonite, Mg(HCO3)(OH)·2H2O, was found to be the dominant degradation product in this study. The experimental data are well in accordance with solubility calculations.