The transformations induced by the anodic oxidation process on the pore structure and surface states of p-type and p(+)-type porous silicon have been studied by Rutherford backscattering spectroscopy, local vibrational mode and electron paramagnetic resonance spectroscopy. Surprisingly the anodic oxidation does not lead to a significant formation of silicon oxide in p-type layers; the oxygen is mainly incorporated in silicon back-bonded configurations without perturbation of the initial H surface passivation. On the contrary, in p(+)-type samples, a uniform oxide layer of a thickness up to 45 Angstrom is formed from the very first stages of anodic oxidation. The influence and correlations between the surface structure and the PL efficiency are discussed.