The gas phase reaction between SiCl4 and NH3 is investigated in the temperature range between 525 and 800 degrees 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 Si-3(NH)(3)Cl5NH2 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 degrees C have an average molar ratio Si:N:CI 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. alpha-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 degrees C.