The effect of both temperature and electrolyte concentration on the formation of porous layers in InP is explored. Pore width, porous layer thickness and porosity decrease with increasing temperature and show a minimum at intermediate KOH concentrations, increasing as concentration is either increased or decreased from that value. The variations in pore width and layer thickness are correlated: thinner pores appear to limit layer thickness, presumably by influencing mass transport throughout the porous network. A three step model of charge transfer at the semiconductor/electrolyte interface is used to explain these observations.