Native oxide interfaces in stain-etched p-type porous Si obtained by room-temperature oxidation of vacuum-annealed (T less than or equal to 650 degrees C) layers have been studied by electron paramagnetic resonance. In addition to the previously evidenced (111) interfaces we have identified (100) interfaces, revealed by the presence of the specific P-bl defects. The (100) interfaces are only observed in the 650 degrees C annealed samples. The 650 degrees C annealing stage is associated with a particular hydrogen effusion peak and the disappearance of the Si-H and Si-H-2 stretching bands. The oxidation of the (111) surfaces proceeds already for lower annealing temperatures (T less than or equal to 300 degrees C). In the 650 degrees C annealed sample, the (100)/(111) interface ratio is estimated to 2/1 and the total oxidised surface to approximate to 36%.