Monofilamentary Nb3Sn superconductors have been made by application of a reaction between Nb6Sn5 powder and niobium. The layer thickness of Nb3Sn shows a parabolic time dependence, implying diffusion-controlled layer growth. The activation energy for diffusion is 268 kJ mol−1. The critical temperature Tc is about 17.5 K and the overall critical current density Jc is about 5 × 108 A m−2at 8 T, both after annealing at 675 °C. These values are comparable with those obtained for so-called ECN-type superconductors, which are based on a reaction between NbSn2 powder and niobium. An advantage of the use of Nb6Sn5 powder in comparison with NbSn2 is the kinetics of growth of Nb3Sn, which allows a lowering of the reaction temperature to 600–625 °C for filament diameters of about 30 μm.