In this study, Sn/GaAs interfaces are studied at both room temperature (RT) and low temperature (LT) (70–100 K) using soft x-ray and ultraviolet photoemission spectroscopies and low-energy electron diffraction. The movement of the Fermi level EF on the RT surface as a function of Sn coverage is initially slow on n-GaAs and fast on p-GaAs. This asymmetry is typically found only at LT for other metal/GaAs interfaces, where it is related to the delayed onset of metallic behavior of the overlayer. In contrast to these other systems, however, Sn/GaAs gives a classic example of Stranski–Krastanov growth at RT. The first two-dimensional layer does not bring EF close to its final position on n-GaAs. At the onset of three-dimensional nucleation (3–4 Å), EF drops abruptly to its final position on n-GaAs. This drop is delayed to ∼5 Å at LT. As for Ag and Au/GaAs, this relatively sharp change in the position of EF appears to be related to the formation of clusters and the appearance of the metallic behavior of the overlayer.