Sodium tungsten bronze powders were synthesized by thermal reduction of a gas/melt system at high temperature. Samples having a cubic structure with different compositions were prepared. The initial melt included Na2WO4, WO3 and 10-40% mol. NaCl while the reducing gas was hydrogen at 750 degrees C. An original mechanism of controlling the powders size and distribution was suggested and discussed. A quantitative novel and simple method to determine the bronze composition based on TGA data was developed. An increase in the NaCl content led to a decrease of the crystals size and improved the powder uniformity. Fine powders, in the 2-5 mu m size range, were synthesized from melt with 40% mol of NaCl. The stoichiometry parameter x of the obtained bronzes ranged from 0.8 to 0.92. An excellent agreement between x values determined by the classical XRD route and the proposed TGA method was demonstrated.