The growth mode, strain state and shape of Ge islands were analyzed in situ, during their growth on Si(001), by combining Grazing Incidence Small Angle X-ray Scattering (GISAXS) and Grazing Incidence X-ray Diffraction (GIXD) measurements. GISAXS measurements provide the detailed evolution of the shape of the grown quantum dots (QDs). GIXD allows the monitoring of the island nucleation and the strain state of the QDs. The shape, size, strain and interdiffusion are found to strongly depend on the growth temperature between 773 K and 923 K. At low T, the 3D islands are small, nearly fully relaxed and do not strain the substrate. At higher T, larger islands with significant Si content are grown, and induce deformation in the silicon Substrate. (c) 2005 Elsevier B.V. All rights reserved.