The structure of the Si(100)-SiO2 interface is investigated at the atomic scale by combining Rutherford ion scattering measurements and theoretical modeling. Ion-scattering experiments are performed in the channeling geometry using ion energies between 0.4 and 2.0 MeV. These measurements are sensitive to Si displacements at the interface between 0.07 and 0.14 Angstrom. To interpret our experimental results, we perform ion-scattering simulations on two realistic model structures of the Si(100)-SiO2 interface. The comparison between experiment and simulation over the full range of considered ion energies supports a Si(100)-SiO2 interface model presenting a disordered pattern of Si-Si dimers in the transition region.