The critical temperatures Tc of Nb3Sn and Nb3(SnGa) monofilamentary wires have been measured as a function of neutron fluence to 1.3×1019 n/cm2 (E≳1 MeV). The bronze overlayer was removed and Tc remeasured. The differences between the bronze‐on and ‐off conditions, a measure of the ’’prestrain effect’’ on the A15 layer, were seen to increase with increasing fluence. This enhancement of the prestrain effect is attributed to buckling of the Nb chains in the vicinity of Sn sites on the Nb chains (antisite disorder), the amount of buckling with the bronze on being greater because of the prestrain. The synergism between irradiation and prestrain is concluded to be a degrading factor in the operation of magnetically confined fusion reactor magnets unless the prestrain (compressive) is completely compensated by the externally applied (tensile) strains.