There are two different models for explaining a homogenization at the atomic level during mechanical alloying. The first model is based on the proposition of local melting as a consequence of ball collisions. The second one focuses on the solid-state reactions. Depending on the homogenization mechanism, the structure evolution during mechanical alloying may be different for individual systems. From this viewpoint we have studied the mechanical alloying of Ag-Cu, Ge-Al, and Fe-Sn systems. The first solid solution prepared from mechanical alloying of the Ag-Cu system has an eutectic concentration; mechanical alloying of the Ge-Al system yields metastable intermetallics and of the Fe-Sn system yields examples of decomposition of the FeSn compound. These results indicate that local contact melting occurs during mechanical alloying.