Si/GaAs(100) interfaces grown at 300, 600, and 770 K on c(8×2)-Ga surfaces prepared by simultaneous argon bombardment at 600 V and annealing cycles at temperatures lower than 800 K have been studied by Auger electron spectroscopy and low-energy electron diffraction (LEED). Si deposition at room temperature up to 8 monolayers (ML) results in disordered Si surfaces. At 600 K the first 2 Si ML grow pseudomorphically to the substrate with the formation of an ordered 1×1 structure at about 0.75 ML. At 770 K Si follows a Stranski–Krastanov mechanism. Up to 0.25 Si ML a (4×2) structure, never reported before, is detected. From 0.25 to 0.75 ML a (1×2) pattern develops which coexists with (2×1) domains above 0.75 ML. This alternation of domains indicates Si dimerization in two orthogonal surface directions. At about 1 ML the (1×2) and (2×1) structures have equal intensities. Above 1.25 ML streaks features appear following the Bragg spot directions. This LEED pattern can be seen even at coverages greater than 4 ML, though with an increasing diffuse background.