To evaluate the reliability in measurements of the thickness of ultrathin gate oxides in the range of 2-9 nm, various techniques based on different methodologies were used for comparison.. The physical thickness was determined with medium energy ion scattering spectroscopy (MEIS), high-resolution transmission election microscopy (HRTEM), and spectroscopic ellipsometry (SE). The physical thickness was compared with the electrical thickness measured with current-voltage (I-V) and capacitance-voltage (C-V) measurements with quantum effect corrections. The physical thickness of amorphous SiO2 layers in the range of 2-9 nm determined with MEIS and HRTEM is in a good agreement with the corresponding electrical thickness from C-V and I-V measurements Within 0.3 nm. For SE, which is the main technique used for in-line monitoring, we observed that it can be used for 2-9 nm ultrathin gate oxides but is more sensitive to the details of the oxide characteristics.