The properties of porous GaP, formed by anodic etching in H2SO4, are described. Pore size, pore density, and the interpore distance depend on the dopant density and the potential at which the sample is etched. In addition, it is shown that at high potential, the GaP passivates as a result of the formation of an oxide layer. These features allow us to grow multilayer structures of GaP with modulated porosity and/or oxide layers. The dissolution of oxide at the base of a porous layer can be used to produce freestanding porous membranes. (C) 2002 The Electrochemical Society.