An increase in stacking fault probability associated with a solute with the same atomic size but a higher valence than the solvent was found by comparing filed Ag and Ag‐6 at. % Al. The stacking fault and twin fault probabilities in ordered Cu3Au, where the atoms differ in size only, were higher than for either pure Cu or Au after deformation at and below room temperature. The stacking fault probability was lower in filings of quenched Cu3Au and Cu‐14.3 at. % Al alloys, than in slowly cooled specimens. There is a close correlation between measured fault probabilities and available data on fault energies and Fermi surface configurations.Twin faults were found in Cu‐base alloys deformed at room temperature as well as at low temperatures.Stacking faults in filings of Cu‐14.3 at. % Al annealed out in two stages. The second stage was associated with recrystallization, with an activation energy similar to that for interdiffusion in this system. Twin faults were more stable than stacking faults.