Surface morphological and compositional evolution during the initial stages of Si growth on Ge(001)2×1 by cyclic gas-source molecular-beam epitaxy (GSMBE) from Si2H6 has been investigated using in-situ reflection high-energy electron diffraction (RHEED), Auger electron spectroscopy (AES), electron energy-loss spectroscopy (EELS), and scanning tunneling microscopy (STM). At 550 °C, single-step-height island growth was observed for nominal Si deposition thicknesses tsi up to ≃ 1.5 ML. The islands were essentially pure Ge which segregated to the surface as H was desorbed. At higher tsi, the Ge coverage decreased, the surface roughened, and two-dimensional multi-layer island growth was observed for tSi up to ≃8 ML above which three-dimensional island growth was obtained. For thick layers (t S: 75 ML), no Ge was detected at the surface.