With all the versatility in structural performance and recent progress in developing magnesium alloys, their Achilles heel remains to be degradation or corrosion property. While applications in mobility demand corrosion protection by all means, bio-medical applications of Mg alloys require well-controlled degradation rates. Meeting these requirements is only possible through the understanding of phenomena on surface–environment interfaces and the characteristics of Mg alloys affecting them. In this study, in situ monitoring during immersion testing along with 3D-optical and scanning-electron microscopy were used for assessing structure-performance characteristics. The effect of alloying with rare-earth metals on the degradation of magnesium has been studied in three model alloys Mg–0.8Nd, Mg–0.2Zr and Mg–2.0Gd using a combination of isothermal calorimetry and pressure measurements. The combination appears to be a powerful method to study corrosion of magnesium. The degradation of the Gd-containing alloy is approximately 100 times that of the other two alloys studied and is associated with the release of heat and hydrogen gas in large quantities. Differences in the morphology of corrosion products on Mg–0.8Nd and Mg–0.2Zr alloy surfaces can be associated with minute variations in the degradation process detected by the developed method.