The mixing of two dissimilar aluminum alloys in friction stir welding (FSW) was investigated using etching. The results show that the materials from the retreating side mixed into the advancing side in rather narrow and elongated bands whereas the materials from the advancing side mixed into the retreating side in the form of thick bands and lobes. A computational method using smoothed particle hydrodynamics (SPH) is introduced as a way to properly describe the complex mixing behavior in FSW.