Friction Stir welding was performed using AZ31-H24 magnesium alloy plates, 0.125 in. (3.175 mm) thick utilizing different welding parameters, that is, travel and rotation speeds. Following processing the welds were cross sectioned and a metallurgical characterization was performed using optical microscopy, and scanning electron microscopy. The evaluation of the weld nugget or "stirred zone" shows evidence of dynamic recrystallization and the start of grain growth in some spots of this region in contrast with the parent metal. Recrystallization phenomenon was also observed in the thermomechanically affected zone (TAZ) even though this was not as noticeable as what was seen in the weld nugget. The mechanical properties of the weld are correlated with the microstructures present in the weld nugget and the thermomechanically affected zone. Corrosion resistance of the weld was assessed using Electrochemical Impedance Spectroscopy (EIS) techniques, but also by performing immersion tests in corrosive environments. Their corrosion performance is being assessed with respect to the processing parameters used and the type of microstructure developed during the welding operation. Copyright © 2006 ASM International®.