By means of diffraction using transmission electron microscopy, the role of stacking fault energy during active plastic deformation of polycrystalline specimens of Cu-Al and Cu-Mn alloys in the low-stability states is revealed. For this purpose, the dislocation and dislocation-disclination substructures in the solid solutions of Cu-Al and Cu-Mn fcc-alloys with the grain sizes within 20-240 subjected to tensile deformation are studied in the concentration ranges of 0.5-14 at.% Al and 0.4-25at.% Mn. An interrelationship between the parameters characterizing the defect substructure and the stacking fault energy is established. The curves of this dependence are compared for different structural-phase states of the alloys. The dislocation substructure (DSS) types of both alloy systems are found to be similar. The only difference is the absence of micro-twinning in Cu- Mn alloys, observed in Cu-Al alloys. The influence of the stacking fault energy on the DSS formation and parameters is analyzed.