The microstructure of oxalic-acid-anodized layers on AlMn0.5Mg0.5 alloy is compared to such layers on aluminum. Differences originate from four types of precipitates occurring in the alloy, forming inclusions in the layers, roughening surface and interface, and modifying typical pore structures of anodized aluminum oxides. A characteristic feature of this modification is the appearance of transverse channels in pore walls. Nanoscaled precipitates are suggested as their origin. Correlation to functional properties such as microhardness and electrical isolation behavior is discussed.