[en] Above T_max∼1050 K, the yield strength σ_t of the commercial γ'-strengthened nickel-base superalloys NIMONIC PE16, NIMONIC 105, and of many similar superalloys decreases drastically. In order to elucidate the physical reasons for this softening effect, σ_t has been measured as a function of the deformation temperature and of the plastic strain rate. Moreover, the configurations of dislocations in deformed specimens have been studied by transmission electron microscopy (TEM). The results are as follows: in NIMONIC 105, the relevant dislocation processes change at T_max: below T_max pairs of dislocations with Burgers vectors of the type (1/2)<1 1 0> shear the strengthening γ'-precipitates athermally whereas above T_max single (1/2)<1 1 0>-dislocations overcome them thermally activated. The effects of changes of the γ'-precipitate dispersion with temperature are negligible. In NIMONIC PE16, however, both mechanisms--changes of the γ'-dispersion and thermally activated dislocation processes--strongly affect σ_t above T_max