The validity of two formation mechanisms of ammonium silicofluoride (ASF), which are proposed to take place when a silicon surface is exposed to the vapor of HNO3/HF acid mixture is investigated. Of the two proposed mechanisms regarding the synthesis of ASF on silicon surface, validity of the first predicting the release of hydrofluosilicic acid (H2SiF6) at the intermediate stage is examined by FTIR spectroscopy and the second mechanism suggesting O-2 release is investigated using the Winkler technique. IR absorbance bands of SiF62+ are observed on the fresh samples prepared at low (1/100) HNO3/HF volume fractions. No significant amount of oxygen is detected during the synthesis of ASF films on silicon surface by dry etching technique. These two observations together provide firmer support for the validity of the second mechanism. (C) 2007 Elsevier B.V. All rights reserved.