We investigated the local atomic structure in GeSi alloy experimentally and theoretically. Extended X-ray absorption fine structure measurements in Czochralski-grown Ge1-xSix bulk alloys showed that the Ge-Ge and Ge-Si bond lengths maintain distinctly different lengths and vary linearly with alloy composition across the whole composition range 0 < x < 1. This is in good agreement with the expectation derived from ab initio electronic structure calculations. The result indicates that the bond lengths and bond angles are distorted with alloy composition in GeSi. (C) 2003 Elsevier B.V. All rights reserved.