Because of its high diffusivity in silicon, aluminum is best suited for deep diffusions often required in high‐voltage‐power semiconductor devices. The ion implantation technique allows the reproducible low dosage doping necessary, e.g., for the new concepts of junction termination systems. The most important drawback of using aluminum as ap‐type dopant in silicon is its low electrical activity after the anneal. In order to obtain a deeper insight into the mechanisms responsible for the loss of the electrical activity, we have studied the states of aluminum implanted into silicon before and after annealing by means of spreading resistance, secondary‐ion mass spectroscopy, transmission electron microscopy, and energy‐dispersive x‐ray techniques. The case study presented here [Czochralski grown (100) silicon, implanted dose 3×1015cm−2, junction depth 6 &mgr;m] reveals that the major source for the loss of the electrical activity is out‐diffusion, i.e., segregation into the native silicon oxide layer and/or evaporation into the vacuum. In addition, the activity is reduced by the formation of aluminum oxide precipitates. The results are discussed in the light of optical studies on the same materials performed previously as well as on the basis of a diffusion model which allows for out‐diffusion. The large rate constant for out‐diffusion indicates that the native oxide layer represents a highly reactive surface for aluminum. From the diffusion model it is possible to calculate an approximate electrical activityA˜(xj) as a function of junction depthxj, which qualitatively reproduces well the observed activityA(xj). This demonstrates that our case study is representative for a large number of samples which were implanted and annealed under widely different conditions. Some technical processes which could possibly enhance the electrical activity are discussed.