Boron nitride presents many interesting properties for high temperature applications: high specific modulus, high electrical resistivity, electromagnetic transparency and strong chemical stability. However synthesis and densification by liquid or gas phase reactions (CVD, CVI or plasma processes) required for high technology applications (thermal shield, protective coating, microwave windows) are difficult to bring into play because material characteristics are strongly dependent on elaboration conditions. Boron nitride ultrafine powder synthesized by gas phase reaction in RF plasma (using NH3 and BCh3 at 800-1400°C) and composites obtained using a rapid densification process (vapor phase infiltration using trichloroborazole compound as precursor) have been studied. In both cases post synthesis heat treatment had to be performed in order to get stabilization of the material, and to avoid its hydrolysis. The stability has been related to the physical characteristics of the material. Thermodynamical studies using Gibbs energy minimization calculations were helpful to determine synthesis and heat treatment conditions.