In this paper, we describe a nondestructive method based on x-ray reflectivity for measuring the thickness of porous silicon layers as well as the interfacial roughness between the porous silicon and the single-crystal silicon substrate. Thickness and interfacial roughness measured using this method compare favorably with values measured using transmission electron microscopy and atomic force microscopy but differ from values obtained by gravimetric techniques for porous silicon layers thinner than 150 nm.