Low‐dislocation‐density copper crystals have been neutron irradiated at low temperatures(T≲50∘K)and transferred to an x‐ray diffractometer at temperatures less than 25°K. Simultaneous measurements of anomalously diffracted x‐ray intensities, the lattice parameter, and the electrical resistivity were made at 4.2°K after successively higher annealing temperatures in an isochronal annealing program; x‐ray measurements were made on approximately 1 × 1 × 0.15‐cm samples, and resistivity measurements were made on wire specimens. The anomalous x‐ray absorption, which is most sentitive to clustering of defects, showed a steady increase beginning at the end of stage I and continuing to about 225°K, the end of stage II. During stage III a small decrease in the absorption was observed. A rather pronounced increase in the absorption occurred in stage IV at about 375°K. The ``clustering peaks'' at 225 and 375°K have been compared with electrical resistivity data, and the results are interpreted in terms of existing annealing models.