We have used transmission electron microscopy to examine in detail how microstructural changes take place in aluminum/copper lines adjacent to tungsten contacts. A structure that could be powered in a probe station was deposited directly on an electron transparent silicon nitride window to allow direct observation of the structure of the line after different intervals of electromigration testing. Observations made on aluminum-4% copper lines show that significant changes in the grain structure surrounding precipitates occur during testing even under conditions where no normal grain growth is observed in the lines. Elecromigration testing at 260°C resulted in grain coarsening and the formation of new diffusion blocking structures in the lines. By making measurements of the volume of individual precipitates as a function of time, the flux of copper through specific segments of the microstructure of the line could be measured. The results indicate that the presence of a single grain boundary can increase the electromigration flux of copper along the line by an order of magnitude. In line segments where a bamboo grain occurred copper diffusion appears to be controlled by diffusion along the Al/oxide interface at the surface of the lines. It is also shown that, in polycrystalline lines, the rate of copper depletion frorr the cathode is largely determined by the rate of dissolution of the precipitates. In mixed structure lines the migration of copper can become quite complex as changes in diffusion paths can occur as precipitates dissolve or grow.