Ag3Sn and Cu3Sn are important intermetallic compounds formed in lead-free solders and dental amalgams. In this paper, we present a study on the microstructural stability in a Ag1.5Cu1.5Sn alloy using a combination of scanning and transmission electron microscopy experiments. The as-cast microstructure comprises the D0(a) phases theta-Ag3Sn and epsilon(1)-Cu3Sn, plus a small amount of eta-Cu6Sn5. There is topotaxial precipitation of Ag3Sn within the Cu3Sn phase, which coarsens with heat treatment at 450 A degrees C. For extended low-temperature aging (1000 h at 100 A degrees C), there is also precipitation of Cu3Sn within the Ag3Sn phase. All the heat-treatments result in the formation of secondary defects in the near-surface region. These defects are dislocations in the Ag3Sn and {011}-type twins in the Cu3Sn. The same defect types are observed in these phases for as-cast samples deformed in compression suggesting that different deformation mechanisms are operating in these isostructural phases.