Nanocrystalline silicon aggregates imbedded in a predominantly amorphous silicon layer have been observed in anodically etchedp‐Si(100) by using valence band x‐ray photoelectron spectroscopy and lattice imaged high‐resolution transmission electron microscopy (XTEM). XTEM has identified the as‐prepared porous silicon to be a mixed phase of amorphous and nanocrystalline silicon, with the nanocrystalline aggregates being randomly dispersed throughout the full thickness of a 1 &mgr;m thick amorphous layer and exhibiting a size distribution from 2 to 5 nm in diameter. The abundance of the nanocrystalline aggregates seems to decrease as the anodic etching proceeds and as the sample is irradiated by x rays at room temperature in ultrahigh vacuum. Valence band photoelectron measurements show evidence for a crystalline‐to‐amorphous phase transition induced by x radiation which may, in part, be activated by photoelectron stimulated hydrogen desorption. The x‐ray irradiated samples also exhibit a significant reduction in photoluminescence yield, possibly caused by a reduction in the density of nanocrystallites. The observed mixed phase porous silicon and the metastability of the nanocrystallites help to explain apparent contradictory descriptions of the nature of porous silicon. ©1995 American Institute of Physics.