Electrolytic anodization was used to form amorphous niobium oxide layers on niobium and V-doped niobium powder compacts which act as dielectric layers, e.g. in niobium-based solid electrolyte capacitors. The microstructure development within the layered structure niobium-niobium oxide was studied by scanning- and transmission electron microscopy in order to investigate the influence of processing parameters. It could be shown that the thickness as well as the quality of the oxide layers on niobium vary in a considerable range, depending on processing parameters. Examination of the influence of heat treatments on the structural properties of the oxide layers revealed remarkable changes at the niobium – niobium oxide interface upon heat impact exceeding 300°C. In contrast, the interface remains stable with respect to the as-anodized sample when annealing was performed at lower temperature.