The mechanical properties ofinsituCu–36 wt&percent; Nb–xwt&percent; Sn superconducting wires are presented where 5<x<20 wt&percent; Sn. For highx, no degradation of critical currentJcis observed for stresses &sgr; up to 700 MPa (∼100 ksi) and a maximumJcoccurs at &sgr;≃450 MPa. Strains of ∼0.6–0.9&percent; are measured at 300 and 77 K for 450 MPa, and ∼1.5–2&percent; for 700 MPa. Cyclic stress data are consistent with a prestress model for composite materials.