This work studies the behavior of the porous silicon layer growth when varying wafer resistivity (p-type) and current density. It shows that under a constant anodizing treatment the porous layer formation regime could be unstable and a transition from a homogeneous nanoporous layer to a filled macroporous layer could occur. The conditions for which the transition takes place are clearly established and it is proved that a slight experimental change modifies drastically the morphology of the porous layer. A low current density and a high wafer resistivity favor the transition but it influences neither the dissolution valence nor the porosity. The transition behavior is compared with reported results and is discussed on the basis of the proposed models for macropore formation. It demonstrates that a complete interpretation of the transition, which leads to the production of macroporous structure, needs to take into account not only the physical and chemical properties of the system but also the configuration of the preparation cell. (C) 2003 The Electrochemical Society.