Many organic materials are damaged and decompose by irradiation of X-rays during XPS measurements. Factors for the decomposition and/or degradation are considered to be X-ray, radiant heat and electrons from the X-ray source, photoelectrons and the inelastically scattered electrons from the sample itself, and so on. We have established the method to evaluate the effect of electrons from the sample by using various inorganic materials as substrates. In this paper, the effect of the thickness of surface oxide layer of silicon wafer on the scission of C-Cl and C-Br bonds in the organic silane monolayers on the wafer surface prepared from chloropropyltrimethoxysilane (CTMS) and bromopropyltrimethoxysilane (BTMS) modifiers has been investigated experimentally. The X-ray source was monochromatic Al Ka, which eliminates the effect of heat and the electrons from the X-ray source. Damaging factors evaluated from the decrease of peak intensity and the fitting to the 1st order reaction rate increased with the thickness of the oxide layer and saturated at about 7 nm. Consequently, the electrons from the sample are considered to be one of the main factors for sample degradation.