Electron diffraction evidence confirming the ordered orthorhombic crystal structure of the gamma phase of the silver-tin system has been obtained, and it has been established by optical metallography that an alloy with a composition corresponding to the dental amalgam alloy formula Ag3Sn (i.e., 26.85 wt % Sn) lies outside the single gamma phase field and in the duplex (gamma + Sn) phase field adjacent to it. Studies of the mechanism of the hardening reaction of single crystals of homogeneous gamma phase alloys with mercury were carried out using both scanning and transmission electron microscopy. Mercury attack occurred preferentially along well-defined planes in the single crystals. Using electron channeling and trace analysis techniques these planes of preferential attack were found to be [010] and [011], and from transmission electron microscopy of thin foils these were shown to be slip bands and deformation twins, respectively. In bicrystals of gamma phase material, preferential attack also occurred along grain boundaries. Similar preferential mercury attack, leading to the development of deep planar intrusions into the gamma phase material, was observed in an experimental dental amalgam prepared from a lathe-cut homogeneous gamma phase amalgam alloy. It is believed that the presence of such features would have important implications for the clinical performance of dental amalgam.