The crystalline structure, the morphology and the chemistry of small Ag particles vacuum-deposited on preheated MgO microcubes were characterized by in situ and ex situ electron microcopy and surface analysis techniques. Observations were made of the nucleation and growth of Ag clusters on MgO surfaces and along surface steps. It is found that Ag particles deposited onto clean MgO crystals have an epitaxial orientation relationship of (100)Ag∥(100)MgO with Ag∥MgO. Prior to air exposure Ag particles are stable under the electron beam but they become unstable upon exposure to air. During electron irradiation on air-exposed samples terraces are formed on the surfaces of MgO cubes, preferentially near Ag particles and at their interfaces with the substrate. The stability of vacuum-deposited Ag particles and the growth of the terraces on MgO microcubes seem to depend on air exposure of the sample.