Measurements on extended dislocation nodes and double ribbons have been made between room temperature and 500°C using transmission electron microscopy. Two low stacking fault energy (SFE) alloys with compositions near the mixed phase region were principally studied; 9 at% Sn (f.c.c.) and 11.9 at% Sn (hexagonal). The SFE in the cubic alloy increased reversibly with increasing temperature from 270 up to 500°C; a corresponding decrease in SFE was observed in the hexagonal alloy. Both alloys showed a substantial irreversible change in faulted defect size upon annealing, comparing results in as‐deformed specimens with those from material annealed above 270°C. The irreversible behavior is attributed to solute pinning of the partial dislocations. The dependence of SFE on composition throughout the f.c.c. and hexagonal phase regions is considered in light of these results.