The influence of the anodization time on some structural characteristics (dissolved mass, maximum porous layer thickness, porosity, crystallite size, etc.) of p-type porous silicon has been investigated. It is shown that chemical dissolution of the porous layer during the anodization must be taken into account in order to correct some of the experimental data. Measurements of the anode potential have allowed to distinguish an early stage of the porous layer formation due to nucleation of pores. Equations based on pore nucleation and growth processes explain well the evolution of the maximum layer thickness with anodization time.