The paper pursues an investigation of the errors associated with the extraction of the dielectric constant (i.e., κ value) from capacitance–voltage measurements on metal oxide semiconductor capacitors. The existence of a transition layer between the high-κ dielectric and the silicon substrate is a factor that affects – in general – the assessment of the electrical data, as well as the extraction of κ. A methodology which accounts for this transition layer and the errors related to other parameters involved in the κ value extraction is presented; moreover, we apply this methodology to experimental CV results on HfO2/SiOx/Si structures produced in different conditions.