A two-step electrochemical process using anodic and cathodic reactions was developed to form size-controlled nanostructures on InP(001) substrates. After anodic formation of a nanopore array, the cathodic decomposition process was applied to reduce the thickness of InP nanowalls. The etching rate of the nanowalls was extremely small and strongly dependent on the cathodic bias and crystal orientations of the wall surface. Wall thickness could be controlled in the range of 10-30 nm by changing the cathodic bias and processing time. (c) 2007 The Electrochemical Society.