Friction Stir Spot Welding (FSSW) has a strong potential to find applications in both the automotive and aerospace industries. At the present time, research efforts are taking place to gain a better understanding of the process, to explore different tool configurations, and to optimize the set of process parameters. In this regard, gaining an understanding of how the material flows under a given set of processing parameters is of particular importance for tool design and to validate numerical models aimed at simulating the process. In this paper, the results corresponding to an experimental study performed using markers are presented to provide insight about the material flow during the plunge phase of the fixed-position refill FSSW process. Copyright © 2005 SAE International.