We have investigated photochemical wet etching of n-type silicon (100) using synchrotron white X-ray radiation. During electroless photochemical wet etching under high flux white X-ray beam, the surface is electropolished. However, when the photon is reduced, the silicon surface becomes porous instead. The pore formation is greatly enhanced when an external potential is applied through a Pt counter electrode. The porous silicon layer exhibits strong photoluminescence signal. (c) 2006 Elsevier B.V. All rights reserved.