Investigations were made of the superconducting transition temperature, Tsub(c), the upper critical flux density, Bsub(c2), and the critical current density, Jsub(c), of Nb/sub 3/Sn layers in filamentary wire in a bronze matrix. The lattice parameter, a/sub 0/ and Tsub(c) of Nb/sub 3/Sn layers in 259-filament wire were determined after removal of the bronze matrix. The microstructure and layer thickness were studied using scanning electron microscopy. The diffusion formation of Nb/sub 3/Sn phase at 1023 K was studied until the complete reaction of the niobium filaments. It was found that the Nb/sub 3/Sn layer begins to form in the manufacturing process during the intermediate annealing at 793 K, and that there is a considerable degradation of critical parameters due to the nonstiochiometry of the Nb/sub 3/Sn phase in layers thinner than 1 ..mu..m.