The anodic oxidation behavior of niobium has been investigated under nearly industrial conditions. High porosity sintered bodies as well as flat niobium sheets were anodized between 20-100 V in 1 wt % aqueous solution of o-phosphoric acid at 65 degreesC, applying current densities between 0.01 and 0.067 mA/cm(2). During formation at constant current, changes in the rate of voltage increase has been observed. Stopping oxidation at such changes in the V(t) curve, the electrical properties, surface composition and in-depth homogeneity of the oxide was analyzed. The formed oxide layers have also been investigated after a heat-treatment in vacuum and in air. Based on the measured composition profiles, the role of the solid suboxide phases [Nb(O), NbO, NbO2] in the oxide growth and in the annealing could be clarified. (C) 2003 The Electrochemical Society.