The influence of various parameters of the electrochemical process on the resulting microstructure of porous silicon layers was studied by spectroscopic ellipsometry. The first parameter, the etching time, is known to determine the layer thickness. In addition, although the current density is kept constant, the microstructure of the layers changes, at least in the top part probed by the light, with increasing etching time. A change in the microstructure was also found upon leaving the samples in the electrolyte for a certain time interval after the end of the electrochemical process. The main effect in both situations seems to be an increase in porosity of the layers. For long etching times or applied illumination with short wavelength light during the etching process the spectra indicate that the diameters of the crystals comprising the silicon skeleton are reduced.