© 2015 The Electrochemical Society. Anodization of highly doped (1018cm-3) n-InP in 2 - 5 mol dm-3KOH under potentiostatic or potentiodynamic conditions results in the formation of a nanoporous sub-surface region. Pores originate from surface pits and an individual, isolated porous domain is formed beneath each pit in the early stages of anodization. Each such domain is separated from the surface by a thin non-porous layer (typically ~40 nm) and is connected to the electrolyte by its pit. Pores emanate from these points along the <111>A crystallographic directions to form domains with the shape of a tetrahedron truncated symmetrically through its center by a plane parallel to the surface of the electrode. We propose a three-step model of electrochemical pore formation: (1) hole generation at pore tips, (2) hole diffusion and (3) electrochemical oxidation of the semiconductor to form etch products. Step 1 determines the overall etch rate. However, if the kinetics of Step 3 are slow relative to Step 2, then etching can occur at preferred crystallographic sites leading to pore propagation in preferential directions.