The catalytic behaviour of porous anodic Al2O3 films with different surface concentrations and lengths of pores vertical to the surface was investigated in the HCOOH decomposition test reaction in which they showed an congruent to 100% dehydrative action. The surface density and length of pores and the position on the pore walls were found to affect strongly the kinetic parameters activation energy, preexponential factor, and total and specific activity at each constant reaction temperature, revealing a strong catalytic heterogeneity of the pore wall surface. Anodic aluminas modified by hydrothermal treatment showed a much different catalytic behaviour, a strong reduction of the pore wall surface heterogeneity, and much higher activities than those of nonmodified aluminas. The promotion factor gave a maximum at a pore length specific for each pore surface density, usually increased with reaction temperature and generally varied between congruent to 3 and congruent to 23. Methods to prepare ultra-active aluminas of designed structure are thus predicted.