We have investigated the laser induced ablation–oxidation process on porous silicon layers having different porosities and thicknesses by non-destructive optical techniques. In particular, the interaction between a low power blue light laser and the porous silicon surfaces has been characterized by variable angle spectroscopic ellipsometry and Fourier transform infrared spectroscopy. The oxidation profiles etched on the porous samples can be tuned as functions of the layer porosity and laser fluence. Oxide stripes of width less than 2 µm and with thicknesses between 100 nm and 5 µm have been produced, depending on the porosity of the porous silicon, by using a 40× focusing objective.