A method has been developed to investigate material embrittlement on the atomistic level. By ion implantation the embrittling agent is introduced into a small volume enclosing a segment of a grain boundary of a thin specimen prepared for transmission electron microscopy (TEM). After allowing the implanted ions to segregate at the grain boundary, the position of the ions is determined by atomic-resolution TEM. In the present study this was applied to the Al(Ga) system. Close correspondence with previous ab initio calculations is found and a model for the initial stages of embrittlement is proposed.