We have used ultraviolet photoemission spectroscopy to study the morphology of the nonreactive materials In, Ag, Sb, and Bi deposited on the GaAs(100) surface. The attenuation of the substrate core levels and the electronic structure of the overlayers yield information on the growth modes and others properties of these interfaces. We find that in all cases the behavior is significantly different from the growth on the more heavily studied (110) surface. In the case of In, we observe isolated In atoms in between large clusters, while on the (110) surface, only strong clustering is observed. Ag forms a larger number of small clusters compared to the (110) surface. This indicates that the mobility of small Ag clusters is lower, and/or the number of nucleation sites is greater on our (100) surfaces. Both Sb and Bi grow in more laminar modes, as opposed to the Stranski–Krastanov mode observed on (110). The Sb film is found to be closer to perfectly laminar. After annealing at 250 °C for 10 min, the Bi film is found to cluster heavily except for a strongly bound single layer. In addition, the resulting Sb thin film is semiconducting up to a thickness of about 100 Å, while the Bi film appears to be semimetallic even at relatively low coverages. We find that the clean surface Fermi level is roughly 0.5 eV from the valence band maximum. However, we will concentrate on the growth mode of the overlayer. Band bending issues will be discussed elsewhere.