An in-depth microstructural characterization was performed on manganese oxide materials that have been produced for electrochemical supercapacitor applications using a novel physical vapor deposition process. Manganese was e-beam evaporated and deposits as a combination of the cubic forms of Mn and MnO with a porous zigzag structure. The electrochemically oxidized sample that is used as the supercapacitor base material is tetragonal Mn3O4. An apparent active layer with increased sodium levels was imaged by STEM, lending some credence to the argument that the pseudocapacitance effect is based entirely on a surface layer of adsorbed sodium. Upon furnace annealing the zigzag structure near the free surface is destroyed and replaced with a columnar oxide layer of cubic MnO and tetragonal Mn3O4. This capping effect ultimately reduces the usable surface area and is thought to account for the reduction in capacitance seen on annealing. (C) 2003 Kluwer Academic Publishers.