The formation of thin nanoporous Si layers on n- and p-Si(lll) was investigated with field emission scanning electron microscopy for preparation in aqueous NH4F solutions with pH 3.2 and 3.8. It was shown that very thin homogeneous nanoporous Si layers can be prepared on both n- and p-Si substrates in a well-controlled manner. The thickness of the nanoporous Si layers was limited by corrosion reactions and could be changed from about 20 nm to 180 nm depending on flowed charge, the concentration of the solution and the anodization current.