The incorporation of Sn atoms as donor impurities in autoepitaxial GaAs films grown from beams of Ga and As4by the process of molecular beam epitaxy (MBE) has been studied in several ways. The sticking coefficient of Sn, as measured directly by modulated beam techniques, is unity over a wide range of growth conditions, and the Sn diffusion rate is low up to the maximum growth temperatures used (820 K). In thick (?1 &mgr;m) films, away from the substrate‐film interface, free‐donor profiles are flat, with a free‐carrier concentration proportional to the Sn flux. Electron mobilities at 300 and 77 K as a function of free‐donor concentration are close to theoretical, assuming a fairly low level of compensation, although there is no evidence for Sn atoms being incorporated as acceptors. There is, however, a surface rate limitation to the incorporation of Sn atoms, so that until a steady‐state surface population of Sn is formed, which can be as large as 0.1 monolayers, the free‐donor density is not constant, but increases exponentially to a steady‐state value. The rate constant associated with this process is strongly dependent on the As4flux, and to a lesser extent on substrate temperature. At constant growth rate (Ga flux constant) and constant Sn flux, therefore, transients are observed in the free‐donor concentration at the beginning of growth, or where the As4flux or substrate temperature are changed during growth. A simple chemical model is shown to describe this behavior.