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Abstract Stacking faults on the (111) planes of several face-centred cubic metals and alloys have been introduced by cold work, and estimates of the stacking fault probability α, have been obtained from changes produced in the Debye-Scherrer spectrum. The faulting probability increases on alloying, from one plane in 300 in copper, to one plane in 25 for some high solute content alloys containing zinc, aluminium, tin or germanium. Both neutron irradiation (5×1019 n.v.t.) and ‘quenched-in’ vacancies have little significant effect on the faulting parameter. Lowering the temperature of deformation increases the faulting probability but in copper the faults ‘anneal-out’ at room temperature after several hours. Line broadening analysis shows that the dislocation density is increased on alloying and also by lowering the deformation temperature. It is suggested that the faulting may be accounted for by the regions between separated half dislocation. Ribbons of stacking faults as given by extended dislocations and ‘infinite’ stacking faults produced by splitting of dislocations at high stresses are both considered and estimates of fault energy made. Some work on segregation of solute atoms to faults is reported with particular reference to the alloys silver-gold and copper-aluminium.