We have studied the microscopic structure of the internal surfaces of porous Si by electron paramagnetic resonance (EPR) and XPS techniques. In particular we have determined the orientation and the passivation states of this surfaces in the as-prepared, aged and thermal annealed states. The EPR results for the as-prepared (p(+), p, p(-)) samples of 75% porosity give evidence exclusively for(lll) surfaces with misorientations relative to the substrate of less than similar to 1 degrees. In the as-prepared state the fraction of oxidized (111) surfaces is less than 1% of the total surface. Aging at ambient conditions, which increases the PL efficiency in p(-) material, modifies the surface structure differently in p(+) and p(-) materials : for the p(+) layers a continuous oxidation is observed over a six-month period, whereas the p(-) layers do not oxidize further. However, amorphous inclusions, the fraction of which increases to 0.6% during six months’ storage, are formed in p(-) material. Both the EPR and the XPS results show that 450 degrees C vacuum annealing strongly increases the oxidized surface fraction : up to 6% and 50% for the p(-) and p(+) materials, respectively, but still showing (111) surfaces only. Contrary to the aged state, now both the p(+) and p(-) materials have disordered inclusions, the fraction of which is up to 40% of the total silicon content. In agreement with the EPR results the XPS results show no evidence for surfaces other than (111).