A mathematical formalism is developed which describes the anodic dissolution of p-type Si in fluoride electrolyte within the first plateau of the electropolishing region. In this region, the silicon surface is coated with a wet oxide layer modelled as a composite medium [SiO2](1-h)[H2O](h) where the local volume fraction of water, h, decreases in the depth of the layer with decreasing distance from the silicon surface. Our theoretical approach based on the resolution of coupled non-linear differential equations involving the It function and the local fluoride concentration fits fairly well typical experimental voltammograms and oxide thickness measurements up to the beginning of the second electropolishing plateau. (C) 2008 Elsevier Ltd. All rights reserved.