A method for producing a multifilament Nb 3 Sn superconducting wire comprising: a) packing a plurality of Cu encased Nb rods within a matrix which is surrounded by an intervening Nb diffusion barrier, wherein the matrix contains a low tin bronze and the matrix contains Cu 0.1-2.5 wt% Sn, and a further matrix on the other side of the barrier remote from the rods, to thereby form a packed subelement for the superconducting wire; b) providing a source of Sn within the subelement; c) assembling the metals within the subelement the relative sizes and ratios of Nb, Cu and Sn being selected such that c1) the Nb fraction of the subelement including and within the diffusion barrier is 50-65% by area, c2) the atomic ratio of the Nb to Sn including and within the diffusion barrier of the subelement is between 2.7 and 3.7, c3) the ratio of the Sn to Cu within the diffusion barrier of the subelement is such that the Sn wt% / (Sn wt% + Cu wt%) is 45%-65%, c4) the Cu to Nb local area ratio of the Cu-encased Nb rods is 0.10-0.50, c5) the Nb diffusion barrier being fully or partially converted to Nb 3 Sn by heat treatment, and c6) the thickness of the Nb diffusion barrier is larger than the radius of the Nb portions of the Cu encased Nb rods; d) assembling the subelements in a further matrix and reducing the assemblage to wire form such that d1) the multifilamentary Nb 3 Sn superconducting wire is comprised of a plurality of the subelements each having a Nb diffusion barrier to thereby form a wire having a distributed barrier design, d2) the Nb portions of the copper encased Nb rods in the final wire are of diameter between 0.5-7 μm before reaction, and d3) the Nb diffusion barrier that is fully or partially converted to Nb 3 Sn by heat treatment is 0.8-11 μm thick before reaction; and e) heat treating the final size wire from step d) to form the Nb 3 Sn superconducting phase. a) packing a plurality of Cu encased Nb rods within a matrix which is surrounded by an intervening Nb diffusion barrier, wherein the matrix contains a low tin bronze and the matrix contains Cu 0.1-2.5 wt% Sn, and a further matrix on the other side of the barrier remote from the rods, to thereby form a packed subelement for the superconducting wire; b) providing a source of Sn within the subelement; c) assembling the metals within the subelement the relative sizes and ratios of Nb, Cu and Sn being selected such that c1) the Nb fraction of the subelement including and within the diffusion barrier is 50-65% by area, c2) the atomic ratio of the Nb to Sn including and within the diffusion barrier of the subelement is between 2.7 and 3.7, c3) the ratio of the Sn to Cu within the diffusion barrier of the subelement is such that the Sn wt% / (Sn wt% + Cu wt%) is 45%-65%, c4) the Cu to Nb local area ratio of the Cu-encased Nb rods is 0.10-0.50, c5) the Nb diffusion barrier being fully or partially converted to Nb 3 Sn by heat treatment, and c6) the thickness of the Nb diffusion barrier is larger than the radius of the Nb portions of the Cu encased Nb rods; c1) the Nb fraction of the subelement including and within the diffusion barrier is 50-65% by area, c2) the atomic ratio of the Nb to Sn including and within the diffusion barrier of the subelement is between 2.7 and 3.7, c3) the ratio of the Sn to Cu within the diffusion barrier of the subelement is such that the Sn wt% / (Sn wt% + Cu wt%) is 45%-65%, c4) the Cu to Nb local area ratio of the Cu-encased Nb rods is 0.10-0.50, c5) the Nb diffusion barrier being fully or partially converted to Nb 3 Sn by heat treatment, and c6) the thickness of the Nb diffusion barrier is larger than the radius of the Nb portions of the Cu encased Nb rods; d) assembling the subelements in a further matrix and reducing the assemblage to wire form such that d1) the multifilamentary Nb 3 Sn superconducting wire is comprised of a plurality of the subelements each having a Nb diffusion barrier to thereby form a wire having a distributed barrier design, d2) the Nb portions of the copper encased Nb rods in the final wire are of diameter between 0.5-7 μm before reaction, and d3) the Nb diffusion barrier that is fully or partially converted to Nb 3 Sn by heat treatment is 0.8-11 μm thick before reaction; and d1) the multifilamentary Nb 3 Sn superconducting wire is comprised of a plurality of the subelements each having a Nb diffusion barrier to thereby form a wire having a distributed barrier design, d2) the Nb portions of the copper encased Nb rods in the final wire are of diameter between 0.5-7 μm before reaction, and d3) the Nb diffusion barrier that is fully or partially converted to Nb 3 Sn by heat treatment is 0.8-11 μm thick before reaction; and e) heat treating the final size wire from step d) to form the Nb 3 Sn superconducting phase.