The conditions under which theNb3Sn intermetallic layer is formed by solid-state reactive diffusion processes in bronzeprocess multifilamentary conductors greatly influence the performance of theconductors. By convention, isothermal heat treatment is used and often causesnon-uniformity of A15 layers formed across the wire. Therefore, characterization andoptimization of the conductor during the reactive diffusion processes is crucial inorder to improve the overall conductor’s performance. In this paper, a differentcharacterization approach and perhaps an optimization technique is presented,namely in situ resistance measurement by an alternating current (AC) method.By treating the components of such multifilamentary wires as a set of parallelresistors, the resistances of the components may be combined using the usual rulesfor resistors in parallel. The results show that the resistivity of the entire wirechanges significantly during the reactive diffusion processes. The development of theNb3Sn layer in bronze process Nb–Sn–Cu–Ta multifilamentary wires at different stages of thereactive diffusion processes has been monitored using measured resistivity changes, andcorrelated with results from DTA, ACS, SEM and EDS.