Nanometer scale porosity is introduced into low-dielectric constant ("low-k") materials to achieve permittivities less than 2.0. X-ray reflectivity (XRR) and small-angle neutron scattering (SANS) are complementary methods for characterizing pore size distributions (PSD). In this paper, the PSD in a polyphenylene low-k material is characterized via solvent vapor porosimetry experiments using both SANS and XRR to probe solvent permeation, and the significant differences in these techniques are described and discussed. Previous work assumed a single-mode PSD, but the solvent porosimetry measurements indicate the presence of a bimodal PSD. A PSD maximum is observed at about 5 angstrom via XRR porosimetry while SANS seems less sensitive to structure at high q (relatively small size scales). A sorption/desorption hysteresis suggests the presence of "ink-bottle" pores in which solvent-filled small pores block the desorption from large pores. A combination of these techniques is also able to describe the structure in a film with partially developed porosity that contains matrix, pores, and porogen.