We use synchrotron radiation based x-ray diffraction at grazing incidence to study the atomic structure of Si-rich beta-SiC(100) 3 x 2 surface reconstruction. The latter includes three different Si atomic planes, in qualitative agreement with the theoretical two adlayers asymmetric dimer model. The measurements provide an accurate determination of the atomic bond, indicating asymmetric Si dimers in the first plane, and an alternating long and short Si dimers subsurface organization in the second atomic plane responsible for the lack of dimers buckling in the first plane, unlike corresponding silicon or germanium surfaces. (C) 2003 American Vacuum Society.