Environmental ellipsometric porosimetry (EEP) was used to characterize nanoporous polystyrene thin films containing cylindrical channels oriented perpendicularly to the surface of the film. These samples were prepared from the selective etching of polylactide from a self-assembled polystyrene-polylactide block copolymer. Isopropanol adsorption-desorption isotherms were obtained from the refractive index variations induced by the adsorption and condensation in the pores. Mean pore size and pore-size distributions were consistent with the values obtained by AFM, SEM, and GI-SAXS. More detailed analysis of the EEP data revealed that the topology of the porous structure was more tortuous than the simple picture of vertically oriented nanochannels might suggest. This was confirmed by preparing inorganic replicas that provided a hard negative copy of the porous network of the film. Microscopy on these replicas confirmed that a fraction of the channels exhibited a U-shape and thus were not perfectly perpendicular to the film surface.