Resistance monitoring has been used to follow structural changes during electromigration in aluminum films. Two stages of migration were found, the first corresponding to gross mass transport, the second to void growth and stripe failure. An activation energy for the first stage was determined to be 0.5–0.6 eV from change‐in‐rate, change‐in‐temperature tests, indicating boundary diffusion. Transmission electron microscopy showed voids existing in areas of thinned aluminum. Corollary work on aluminum stripes on NaCl substrates showed nonadhesion to be a strong contributor to void formation, suggesting the possibility that the thinned regions were caused by hot spots at sites of nonadhesion.