The wet-chemical acidic treatment of silicon wafer surfaces is very important in photovoltaic, microelectronic and further industries. Recent works report on new mixtures for acidic anisotropic etching mixtures based on hydrofluoric acid HF and hydrochloric acid HCl with an added oxidant. The aim of this work was to get an insight into the reactions during the etching process of silicon in the system HF-HCl-Cl-2. The etching mixtures, gaseous reaction products, as well as the generated silicon surfaces were investigated by F-19, Si-29, and Cl-35 NMR, ion chromatography (IC), iodometric titration, FT-IR spectroscopy, diffuse reflectance FT-IR spectroscopy (DRIFT) as well as scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDX). A reaction scheme for the anisotropic dissolution of silicon in chlorine containing aqueous HF-solutions is proposed, which involves dissolved Cl-2 as the oxidizing agent, coordination of fluoride/chloride ions and formation of a hydrophilic surface. These steps are similar to the well known alkaline anisotropic etching of silicon. (C) The Author(s) 2017. Published by ECS.