A comparative study of the anodic oxidation of tantalum plates and high porosity sintered powders (particle size 1 mum) was carried out. The experimental results show a similar behaviour during the oxidation at a constant current but great differences at a constant voltage. This may be attributed to a change in the ionic conduction mechanism owing to the curvature of the oxidation surface. Although the diffusion effects may be ignored in the oxidation of tantalum plates they constitute the basis of the ionic current during the oxidation of sintered powders with small particle size.