The precipitation behavior of evaporated Al/Si films with up to 1.8% Si is studied by microscopy as well as by resistance measurements after isochronal and isothermal aging. After deposition of the film most of the Si is precipitated. Temperature treatments above 300°C cause the solution of Si into Al. Aging of a supersaturated solution results in precipitation on grain boundaries at a rate that is much greater than in bulk Al/Si. The precipitation is described as a two‐dimensional diffusion‐limited process. A theoretical expression for the rate constant permits the determination of the diffusion coefficient of Si in Al. This is much greater than in bulk Al/Si and has an activation energy of about 0.85 eV. In view of the small vacancy concentration in the film, these results are explained by diffusion along dislocations, in agreement with electron micrographs revealing a dislocation density of about 1010cm−2.