A novel technique for the formation and lift-off of thin porous silicon films from starting substrates in a single step by electrochemical etching in hydrofluoric acid-based solutions is described. Lift-off or separation of porous silicon (PS) film occurs under specific sets of current density and HF concentration, which also determines the PS film thickness that can vary from a few micrometers to a few tens of micrometers. A model based on a diffusion-limited mass transfer of HF molecules from the bulk of the solution to the point of reaction at the pore tip is proposed to explain the lift-off phenomena. Based on this an expression for expected PS film thickness and separation time as a function of current density and hydrofluoric acid concentration has been derived. The experimental results are in agreement with the proposed model. (C) 2004 The Electrochemical Society.