When measuring the thickness of ultrathin overlayer films using X‐ray photoelectron spectroscopy (XPS), accurate values of the reference intensity ratio (R0) and the effective attenuation length (L) are essential. By definition, R0 is the peak intensity ratio for an overlayer and the substrate in “bulk” phases. Two issues need to be addressed in experimental determining R0 for ultrathin films: (i) How might a contamination layer on the sample used for measuring peak intensities impact R0? And (ii) do differences in the structure or chemistry of an ultrathin film make it inappropriate to determine R0 using bulk forms of the overlayer? In this study, we demonstrate the experimental determination of the R0 for an ultrathin HfO2 film on a Si(100) substrate with a 2 nm SiO2 layer. The values of R0 were determined using (i) the bulk materials of the HfO2 film and substrate and (ii) the ultrathin HfO2 films after different cleaning treatments. The results show that the R0 determined by the ultrathin films is higher than that determined by the bulk materials. Also, keeping the same level of carbonaceous contamination on the sample surface by cleaning as much as possible is essential for an accurate experimental determination of R0. In addition, the effective attenuation length was obtained using samples with known thicknesses measured by X‐ray reflectometry. The thicknesses and uncertainty budget of the ultrathin HfO2 films were then evaluated.