The characteristic mechanical behavior of bulk Ge1-xSix alloys with compositions x=0–0.4 grown by the Czochralski technique is investigated as a function of the composition and temperature by means of compressive deformation and is compared with that of elemental Ge and Si semiconductors. Both the yield stress and the flow stress at any strain increase with increasing Si content in the composition range investigated. The yield stress of the GeSi alloy is temperature insensitive at high temperatures, showing that the flow stress of an alloy semiconductor has an athermal component that is absent in elemental or compound semiconductors. The hardening mechanism in alloy semiconductors is discussed. © 1996 American Institute of Physics.