In continuation of our earlier investigations on the microstructures of cold‐worked Cu‐(Sn,Ni,Mn)‐Zn ternary alloys by x‐ray diffraction line profile analysis [J. Appl. Phys.54, 6652 (1983);ibid.48, 3560 (1977);ibid.56, 1213 (1984)] the present work with Cu‐Mn‐Si alloy system in the fcc phase has been undertaken to elucidate the effects of Mn and Si, when present together, on the cold‐worked substructure of this system. From detailed analyses of peak shift, peak asymmetry, and Fourier line shapes, quantitative estimates of the related microstructural parameters, namely, intrinsic (&agr;’), extrinsic (&agr;‘), and twin fault (&bgr;) probabilities, domain sizes, rms strain, dislocation density, and stacking fault energy (for pure Cu), have been made. The results indicate, as in the earlier cases, increased presence of intrinsic stacking faults (&agr;’)—primarily responsible for the observed peak shift and domain size broadening with relatively small presence of extrinsic faults (&agr;‘) and complete absence or very small presence of deformation twins (&bgr;) responsible for the observed asymmetry in the profiles. Unlike earlier observations, the presence of transitional solute Mn along with nontransitional Si as another solute appears to have a pronounced effect on the stacking fault probability of the present Cu‐base alloys in the fcc phase.