Transverse cross-sections of Nb-Ti wire composites after final α-Ti precipitation heat treatment (10-480hr/420°C) were examined and analyzed in the filament-matrix region with scanning electron microscope-backscatter electron imaging (SEM-BEI) and electron micro probe analysis (EMPA). SEM-BEI micrographs were image analyzed to quantify the average effective α-Ti precipitate diameter, d*, and volume fraction of α-Ti as a function of distance into the filament. Cu concentration profiles were found in the same regions by EMPA. The compositional results show trace amounts of Cu (>0.1 at%) interdiffused up to 50 μm into the Nb-Ti filaments. The interdiffused Cu at the interface increased α-Ti nucleation and thus increased α-Ti volume fractions by 50% relative to the nominally pure Nb-Ti at the center of the filament for short heat treatment times (30 hours of total heat treatment time).