The etching of n-type silicon in hydrofluoric acid solutions containing active oxidising agents has been studied. The effect of different parameters on the etch rate of silicon in these solutions has been examined. Based on electrochemical experiments Coupled to in situ real-time measurements by infrared spectroscopy, the evolution Of Surface chemistry during etching of Si has been investigated. It has been found that dissolution of silicon may take place with and without oxide formation on the electrode surface, depending on the composition of etching solution. The transition between conditions giving rise to porous silicon generation and those leading to surface polishing was analysed. when the concentration of oxidising species is increased. In the polishing regime, oxide thickness was estimated from the intensity of the Si-O absorption band.