Ultrasonic metal welding is a joining method, which sees widespread application in the manufacturing of similar as well as dissimilar joints of electrical conductors, e.g. stranded wires and terminals for usage in wire harnesses. As joining takes place below the melting temperature of both joining partners it is classified as a solid-state welding technique. Due to this, the formation of brittle intermetallic compounds at the interface which exhibit poor conductivity can be prevented to a large extent. Additionally, due to its energy efficiency, short process times and the avoidance of process gasses or additives ultrasonic metal welding is being adopted in more and more industrial branches. Although this technology features many advantages over conventional joining methods it is highly sensitive to the surface conditions of the joining partners. In these investigations focus lies on the surface conditions of the stranded aluminum wires used for joining with copper terminals. Five different states of the stranded aluminum wires were produced and ultrasonically welded to copper terminals. The produced joints and single strands were examined via light as well as scanning electron microscopy, x-ray photoelectron spectroscopy and tensile testing. Inter alia the examinations have shown that artificially generated surface layers of AlO(OH) are not broken up by the ultrasonic oscillations thus impeding with inter-strand connection and adhesion of the aluminum wire to the copper terminal, which led to a decrease of joint strength by 90 %.