Abstract: An electroless etching process. The process produces nanostructured semiconductors in which an oxidant (Ox1) is deposited as a metal on a semiconductor surface and used as a catalytic agent to facilitate reaction between a semiconductor and a second oxidant (Ox2). Ox2 is used to initiate etching by injecting holes into the semiconductor valence band as facilitated by the catalytic action of the deposited metal. The extent of reaction is controlled by the amount of Ox2 added; the reaction rate is controlled by the injection rate of Ox2. The process produces high specific surface area and/or hierarchically structured porous Si with higher and controllable yield. In addition, the ability is demonstrated to vary the pore size distribution of mesoporous silicon including producing hierarchically structured mesoporous silicon with more than one peak in the pore size distribution. In principle, the process applies to any semiconductor onto which metal can be deposited galvanically.