A selective solvent induced surface reconstruction of thin films of an asymmetric diblock copolymer of polystyrene-b-methyl methacrylate) was investigated by grazing incidence small-angle X-ray scattering, X-ray reflectivity, and atomic force microscopy. It is quantitatively shown that the selective swelling of the minor component results in the formation of a nanoporous film where the initial hexagonal array of the copolymer is preserved without affecting the lattice constant, 40.7 nm. However, the film thickness increased by similar to17%, and the average electron density decreased by similar to20%. The electron density profiles obtained by fitting the X-ray reflectivity indicate that the pores penetrate through the entire film and that the pores are oriented normal to the surface.