THE preparation of highly porous materials by sol-gel processing followed by supercritical drying was pioneered by Kistler in 1931(1). These materials, called aerogels and comprising a void fraction of more than 90%, are currently a focus of technological interest(2). Here we show that supercritical drying can be used to prepare highly porous silicon, a material of particular interest because of its luminescent properties(3). Porous silicon is prepared conventionally by electrochemical etching, but using supercritical fluid extraction allows us to obtain porosities that would otherwise lead to collapse of the network during drying. Electron microscopy reveals that our materials have a crystalline, columnar structure, and gravimetric and ellipsometric analysis indicates that the porosity exceeds 95%. Our silicon aerocrystals exhibit strong photoluminescence. We predict that supercritical drying should greatly improve those properties of porous silicon films that are critical for optoelectronic applications.