``Particle'' or domain sizes and angular misorientations between domains change during the annealing of cold‐worked crystals, leading to primary and secondary extinction changes in the x‐ray diffraction lines. It has been found that domain sizes are too small in cold‐rolled copper to produce significant primary extinction. An attempt was made to avaluate the secondary extinction changes in terms of Darwin's equation. The invalidity of the commonly used form of this equation was shown. With a dislocation model for the substructure it was possible to evaluate the integrated intensity ratioI/Is, with‐to‐without extinction, in terms of domain sizes which were determined by Fourier analysis of line shapes. It was found that microstrains annealed out prior to recrystallization but that secondary extinction began to change significantly only after the domain size had increased to about 700 Å. Thus there must be a considerable decrease in dislocation density prior to the misorientation changes which accompany secondary extinction increases in copper. It was found possible to predict the strength changes during annealing of copper from the secondary extinction changes associated with x‐ray integrated intensity; this is of considerable practical importance for thin sheets.