A new approach for the selective chemical modification of porous silicon surfaces with organic monolayers is reported. The organic layers are first covalently grafted onto (100) surface of hydrogen terminated silicon via thermal hydrosilylation of alkene or functionalized alkene molecules yielding stable monolayers on flat Si surfaces. The functionalized silicon surfaces are then electrochemically etched in a hydrofluoric acid solution. [GRAPHICS] The resulting porous films feature the possibility of both further chemical modification of the fresh internal surface and a discrimination between internal and external surface chemical properties. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim