The interaction of copper with dislocations was studied in silicon?doped gallium arsenide by means of cathodoluminescence, analytical, and transmission electron microscopy. Several structures of defect complexes or microdefects surrounding the dislocations were found depending on the diffusion temperature and cooling rate. The results could be explained by considering the local nonequilibrium of intrinsic point defects induced by Cu in- or outdiffusion. The appearance of a bright or dark dislocation contrast in the cathodoluminescence pictures is related for different diffusion conditions i) to the enrichment of copper acceptors at dislocations, ii) to the distribution of silicon-vacancy complexes, and iii) to non-radiative recombination at Cu?As precipitates or clouds of small dislocation loops.