X-ray synchrotron topography was used to study the structural features of porous silicon (PS) layers formed by p(+) (boron)-type silicon anodization within localized areas. Selective anodization was obtained by using a masking film (Si3N4) with good chemical inertness to the anodic dissolution or a surface layer of different doping type and level resulting from phosphorus implantation. The overall structural picture of the PS/substrate systems obtained by the two different masking procedures was built up by combining different topographic methods. The samples were oriented both in Bragg (reflection) and Lane (transmission) geometry to diffract white or monochromatic radiation with extended (projection topography) or limited (section topography) beam widths. By comparing the effects of the two masking types on the anodization process, the implantation method is unquestionably preferable because of the lower stress at the window edges, the better lateral porosity homogeneity inside the windows, and the flatter interface between porous silicon and substrate.