The combination of angle-resolved X-ray photoelectron spectroscopy (ARXPS) and a modified Levenberg-Marquardt (LM) fit procedure has been used to study a native oxide layer on a clean Si(100) substrate. Numerical calculations show that with an aperture of 3 o or 9 o of the electron analyser, the photoelectron takeoff angle should not exceed 80 o or 70 o, respectively, as compared to normal takeoff angles. At larger photoelectron takeoff angles, the effect of the aperture on the photoelectron energy distribution may not be neglected. We show how absolute ARXPS measurements in which the same XPS feature is considered at several electron takeoff angles are an alternative for relative ARXPS film thickness measurements, avoiding large errors in the quantitative results. Models for the composition and thickness of the oxide layer have been developed. Also, the errors in the parameters of these models have been calculated. It can be concluded that the native oxide layer on silicon is 27 + 1 (:t: 5%) ,~ thick and that the ratio of the silicon atom concentration in the substrate to that in the native oxide layer is 3.7 :t: 0.3 (:t:8%), values that agree well with the literature. This report shows that the combination of ARXPS and a LM fit procedure is well suited to study ultra-thin layers and gives reliable results.