Superplastic behavior of a solution treated and friction stir processed (FSP) AZ91C alloy is studied. These studies are conducted in the temperature range of 300-375 A degrees C and strain rates (SRs) in the range of 1 x 10(-4)-3 x 10(-3) s(-1). Microstructural stability of the FSP alloy is also studied in comparison to the AZ31, AZ61, and AZ91 alloys processed by various routes. High SR sensitivity in the range of 0.33-0.39 and grain size stability till 350 A degrees C is observed for the FSP alloy. The FSP AZ91C alloy showed better thermal stability in comparison to AZ31 and AZ61 alloys. Kinetics of superplastic deformation of the FSP alloy is found to be slower as compared to AZ31 and AZ61 alloys processed by various routes, which is due to the presence of significant amount of second phase precipitates, such as, beta-Mg-17(Al,Zn)(12), Mg2Si, and Al8Mn5 in the FSP alloy. However, these precipitates contributed for better thermal stability of the microstructure of FSP AZ91C alloy.