During leaching tests of different Aurubis anode slimes, decreased copper, and nickel leaching yields were observed for one particular slime. This anode slime had a higher copper, nickel and antimony content than the others. To identify the reason for this behavior mineralogical investigations of the slime before and after leaching were performed. These showed only low amounts of copper and nickel oxides but considerable amounts of a CuNiSbO species. There have been several studies on the species called kupferglimmer, Cu3Ni2–xSbO6–x. It is known to have a refractory nature, which poses a challenge for further anode slime processing. Higher amounts of residual copper and nickel in the leached slimes lead to capacity problems and difficulties during silver and precious metal processing. Thus, in this study the conditions in anodes leading to specific anode slimes of certain compositions and mineralogies were investigated together with the resulting leaching behavior. When the higher antimony levels were accompanied by higher lead levels in the anodes, the copper content of the slime decreased, and leaching yields for copper and nickel improved.