In copper diffused gallium arsenide cathodoluminescence investigations were combined with analytical and transmission electron microscopy in order to interpret the occurrence of bright or dark dislocation contrasts. Various structures of Cu agglomerations were found in dependence on diffusion temperature and cooling rate. Bright dislocation contrasts are related to the enrichment of solute Cu_Ga acceptors, wheras dark contrasts are mainly due to non-radiative recombination of Cu-As precipitates or clouds of small dislocation loops.