We demonstrate the use of contrast matched resonant soft X-ray scattering (RSoXS) for studying the morphology of mesoporous polystyrene-block-polyethylene-block-polystyrene (SES) copolymer membranes. The mesoporous membranes were obtained by blending the SES copolymer and polystyrene (PS) homopolymer to obtain a nonporous membrane, followed by selective dissolution of the PS. If the PS homopolymer chains are initially located within the PS microphase of the SES copolymer, then one obtains a porous film wherein the pores are lined with PS. We refer to this as the templated morphology. The membranes are thus composed of three phases; voids, PS, and PE. Conventional techniques such as small-angle X-ray scattering (SAXS) and scanning electron microscopy (SEM) only distinguish between the polymer and the voids. The main advance in this paper is to show that microphase separation between PS and PE can be studied by contrast-matched RSoXS in spite of the presence of voids. Under certain circumstances, we obtain mesoporous membranes that are not templated by the SES copolymer. We show that RSoXS can be used to distinguish between templated and nontemplated, mesoporous films.