The results of reaction between Nb(Ta) filaments and a Cu(Sn) matrix have been studied for filament compositions ranging from 0 to 34 at pct Ta. Microprobe measurements show that the Ta is fully incorporated into the reaction layer, producing (Nb,Ta)3Sn ternary A-15 compounds. High Ta concentrations lead to a decrease in reaction kinetics. Superconducting critical temperatures decrease with increasing Ta content more rapidly than reported earlier for powder samples. However, small Ta concentrations produce a decreased field dependence of critical currents and an increase in critical currents at high fields. These effects are believed to result from increased critical fields produced by a decreased electron mean free path, and may prove to be of technological significance for the production of high-field superconducting magnets.