Multifilament conductors have been fabricated using niobium tubes with a sheathed tin‐copper alloy inside and pure copper matrix outside. These composites have good workability, cold drawn without any intermediate heat treatment. Reactions and their effect upon superconducting properties have been checked for temperatures in the 625–800 °C range and various inner filament sizes ranging from 18 to 1154 &mgr;m. The Nb3Sn layer thickness grows ast0.8in most of the reaction time, which cannot be explained by an ordinary diffusion process. The pinning force is found to be inversely proportional to the square root of the grain size measured through a fractographic technique in the 0.1–0.3‐&mgr;m range. Also, the activation energy of the Cu/Sn‐Cu diffusion couple is obtained as 47 kcal/mol from the reaction‐temperature–time relation. These results have been discussed and compared with others.