Superconducting critical temperaturesTcand magnetic fieldsHc2, lattice parametersa0, and chemical compositions were measured for ‘‘bulk’’ layers (∼6 &mgr;m or greater) of ‘‘pure’’ and alloyed Nb3Sn which were made by the bronze process. The values ofTc,a0, and the composition of pure Nb3Sn layers were ∼18 K, 0.52900±0.00005 nm, and 25±0.5 at. % Sn, respectively, independent of heat‐treatment temperature (between 650–780 °C) and of the bronze composition, as long as the thickness of the layers was greater than ∼6 &mgr;m. Small additions of Ti (∼1 at. %) or Ta (∼3 at. %) slightly increased the value ofTc(by ∼0.2–0.4 K) above that for pure Nb3Sn. However, additions of larger amounts of these elements or addition of other transition elements (V, Zr, and Mo) significantly decreasedTc. Also, small additions of these elements significantly increasedHc2. Specifically, the largest values ofHc2(∼27 T at 4.2 K) were obtained for Nb3Sn layers containing ∼1.5 and ∼4 at. % of Ti and Ta, respectively, compared with a value for the pure Nb3Sn of 23.5 T at 4.2 K. The value ofa0decreased with all of the alloying additions; these variations can be explained qualitatively by several models for the lattice parameter of A15 compounds, but none of them can quantitatively predict the variations. In one system, (Nb,Ti)3Sn, values of the normal‐state resistivity just above the transition temperature were measured: adding 3 at. % Ti raises the value to ∼55 &mgr;&OHgr; cm from the value of 10–15 &mgr;&OHgr; cm for pure Nb3Sn. This increase in the resistivity is thought to be a primary reason for the increasedHc2for this system as well as the other types of alloyed Nb3Sn which were studied here.