A new method for quantitative etching of the poly(dimethylsiloxane) block in polystyrene-poly(dimethylsiloxane) (PS-PDMS) block copolymers is reported. Reacting the block copolymer with anhydrous hydrogen fluoride renders a nanoporous material (NPM) with the remaining glassy PS maintaining the original bulk morphology. 1H NMR, mass difference, size exclusion chromatography, and X-ray photoelectron spectroscopy were used to characterize the materials before and after etching. NPMs containing spherical and gyroid cavities were prepared, as ascertained by small-angle X-ray scattering. This is the first report on block copolymer-based NPM films of millimeter thickness containing secluded spherical holes. Surface images by AFM and SEM are consistent with the SAXS findings.