The analysis of the ultrasonically welded zone between aluminum alloys is taken as an example of the possibilities of analytical TEM to gain an insight into the phenomena occurred at the inner boundary surfaces of contacting metals during the ultrasonic welding, It is shown that combined TEM studies (imaging, diffraction, chemical analysis) of the welded seam allow to thoroughly characterize such a "buried" layer as the ultrasonic welded zone and to reconstruct, thereby, the physical and chemical processes responsible for its formation. (c) 2005 Elsevier B.V All rights reserved.