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The gas phase reaction between SiCl4 and NH3 is investigated in the temperature range between 525 and 800°C at atmospheric pressure and at conditions typical for powder synthesis. By means of mass spectrometric in-situ measurements it was possible to detect the gaseous compounds H2NSiCl3, H2NSiCl2NH2, Cl3SiNHSiCl3, NH2Cl2SiNHSiCl3, (SiCl2NH)3 and Si3(NH)3Cl5NH2. The reactions taken place in the gas phase are very fast and result in the formation of a fine, chlorine containing product. Powders sampled at a reaction temperature of 800°C have an average molar ratio Si: N: Cl of 1: 1, 33: 0.28. Based on the proved gaseous intermediates and the composition of the powders reaction paths resulting in the formation of powders are derived. α-Si3N4 powders with a high sintering activity are obtained after thermal dechlorination of the synthesis products in ammonia atmosphere followed by a crystallization process between 1200 and 1500°C.