A critical evaluation is made of the procedure and the important parameters involved in estimating the stacking‐fault energy &Ggr; from x‐ray diffraction measurements of deformed materials. The published data for the Ag‐In and the Cu‐Zn system are carefully analyzed and compared with results obtained by transmission electron microscopy. For deformed filings, the dislocation density, and hence the mean square strain ⟨&egr;2⟩hkl, vary with composition, as does the stacking‐fault probability &agr;. Since &Ggr;∝&agr;−1⟨&egr;2⟩hkl, the value for the proportionality constant must be known to obtain &Ggr;. This requires a knowledge of the dislocation configuration, principally in terms of barriers and pile‐ups of extended dislocations. Within the limits of uncertainty of this information, an assessment of the proportionality constant can be made so that reasonably good consistency with transmission electron microscopy results is obtained. In this way, estimates (good to ±25%, probably) of &Ggr;=18 erg/cm2for pure silver and &Ggr;=65−74 erg/cm2for pure copper could be calculated.