In this work, the degradation behavior of AZ91 magnesium alloy in five solutions including phosphate buffering solution (PBS), 0.9% NaCl solution, simulated body fluid (c-SBF), Hank's solution, and Dulbecco's modified eagle medium (DMEM) are systematically investigated. Our results show that AZ91 magnesium alloy exhibits a dramatically different degradation behavior in the five solutions. The concentration of buffering agents and the content of hydrocarbonates in test solutions primarily affect the degradation performance, especially the degradation behavior during the early stage. In DMEM and c-SBF with higher contents of buffering agents, the sample shows much more negative corrosion potentials compared to those in Hank's solution, 0.9% NaCl solution, and PBS. The initial degradation rates in DMEM and c-SBF are similar and much higher than those in PBS, 0.9% NaCl, and Hank's solution. Rapid reaction of OH- with the large amount of buffers in DMEM and c-SBF dramatically promotes transformation from Mg to Mg2+, constituting the main reason for the high degradation rates. Distinct corrosion morphology and corrosion products are present on the samples exposed to the five test solutions. Therefore, the proper selection of test solution with similar buffering capability and similar ingredients as biological fluids is imperative to the investigation of the degradation behavior of biomedical magnesium alloys. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3421651] All rights reserved.