Scanning electron microscopy, atomic force microscopy, and Raman spectroscopy were used to characterize the microstructure of photoluminescent porous silicon (PS) layers formed by the anodic etching (HF:H2O:ethanol), at various current densities, of p-type (100) silicon wafers possessing resistivity in the range 1-2 OMEGA cm. Existing models for the origin of luminescence in PS are not supported by our observations. Cross-sectional as well as surface atomic force micrographs show the material to be clumpy rather than columnar; rodlike structures are not observed down to a scale of 40 nm. A three-dimensional model of the mesostructure of porous silicon is discussed. Room-temperature Raman scattering measurements show no evidence for a-Si:H or polysilanes and the material reported here is composed of 10 nm roughly spherical Si nanocrytallites rather than 3 nm wires postulated in standard quantum confinement models.