The present study demonstrates that the phenomenon of non-thickness limited (NTL) growth of anodic films in a phosphate/ glycerol electrolyte at 453 K on tantalum and aluminium is due to the formation of porous films. For both tantala and alumina films, the resultant morphology comprises cells of material orientated approximately normal to the metal/film interface, with each cell containing a central pore of typical diameter in the range 3-15 nm and a barrier layer of film material at the base of the pore. The total porosity is about 5%. The hardness of NTL tantala films, determined by nanoindentation, was 5.1 GPa, reasonably similar to that of conventional anodic tantala.