Formation of Kirkendall voids is demonstrated in the Ge underlayer of reverse step graded Si 1−x Ge x /Ge buffer layers grown on Si(001) using reduced pressure chemical vapour deposition (RP-CVD). This phenomenon is seen when the constant composition Si 1−x Ge x layer is grown at high temperatures and for x0.7. The density and size of the spherical voids can be tuned by changing Ge content in the Si 1−x Ge x and other growth parameters.