© The Electrochemical Society. Pores formed anodically in InP at different temperatures, electrolyte (KOH) concentrations, carrier concentrations and current densities exhibit significant pore width variations. The pore width decreases as the temperature, carrier concentration or current density are increased. The pore width also decreases when the KOH concentration is increased up to 9 mol dm-3, but increases slightly as the concentration is increased further. These pore width variations are explained by a three-step model for pore formation based on competition in kinetics between the different steps in the etching mechanism. The variation of pore width with current density is explained explicitly in terms of the crystallographic etching mechanism and this is supported by observation of the different crystallographic features of the pore cross section at different current densities.