Magnesium alloys have been proposed as suitable biomaterials for resorbable implant applications. Although a substantial amount of research has been performed on the use of Mg alloys for biomedical applications, there are few studies employing micro-electrochemical techniques for characterizing these materials in physiological environments. The unique capabilities of the scanning electrochemical microscope (SECM) make this a powerful tool for in situ study of the localized corrosion of Mg alloys in physiological fluids. The work presented will assess the initial stages of electrochemical corrosion attack of AZ31 and AZ91D Mg alloys, and compare their performance in simulated body fluid (SBF). SECM measurements in amperometric operation modes were carried out in order to characterize the alloys surface during corrosion. Substrate Generation/Tip Collection (SG/TC) mode was used to spatially resolve regions of hydrogen flux related to cathodic sites while Feedback mode was used to distinguish between conductive/insulating regions on the passive layer. These results show intensive electrochemical activity on substrate surfaces at short exposure times of up to one hour, demonstrating rapid pit nucleation and formation of nearby, surrounding corrosion products. These results provide useful information on the localized hydrogen evolution process at Mg alloy surfaces under physiologically relevant conditions.