Niobium oxynitrides were prepared by nitridation of niobium pentoxides in flowing pure ammonia, at atmospheric pressure. Between 900 K and 1100 K a face centred cubic phase was synthesized. Nitrogen content and unit cell parameters of this oxynitride increase with the temperature of preparation. Above 1200 K a hexagonal phase similar to epsilon-NbN was observed. Optimization of the nitridation process was studied to produce oxynitrides with high specific surface areas. The influence of parameters such as structure and composition of the precursor oxide, final temperature of nitridation and composition of the gaseous mixture was investigated. Nitridation of Prolabo Nb2O5 at 900 K resulted in a face centred cubic oxynitride with 61 m2 g-1. The mixture of M-Nb2O5 with a low weight percent (5 wt.-%) of oxynitride was found to facilitate the reduction-nitridation process of the precursor and led to an oxynitride with 80 m2 g-1. The pore size distribution was mainly between 3 and 4 nm. The thermal stability of the oxynitrides was studied by temperature-programmed desorption (TPD) in vacuum. The various species adsorbed on the solid surface were identified and their desorption spectrum established. TPD experiments enabled to check if the pretreatment was able to clean the active surface.