Delineation of crystalline defects in silicon substrates and thin films by chemical etching in combination with light optical microscopy is a well established method for quality control. It is still the workhorse for a quick and simple evaluation of defect types and area densities. Most of the etching solutions used today have two disadvantages. They contain hexavalent chromium which is highly toxic and they are not suitable for application on thin silicon films. A new class of chromium free etching solutions containing organic peracids was developed. These solutions are able to reveal different crystalline defects like oxidation induced stacking faults (OSF), dislocations and vacancy agglomerates (D-defects)in SOI and sSOI.