We investigate the systematic error due to neglect of elastic scattering of photoelectrons in measurements of the thicknesses of thin films Of SiO2 on Si by x-ray photoelectron spectroscopy (XPS). Calculations were made of substrate Si 2p photoelectron currents excited by Mg and Al K alpha x rays for different SiO2 thicknesses, different angles of photoelectron emission, and three representative XPS configurations using an algorithm based on the transport approximation. We calculated practical effective attenuation lengths (EALs) from changes of the computed Si 2p photoelectron cur-rents. These EALs were less than the corresponding inelastic mean free paths by between 6.5% and 9.4%, with the difference depending on the x-ray source, the specific range of SiO2 film thicknesses under consideration, the XPS configuration, and the range of photoelectron emission angles. Useful average values of the EAL were found for emission angles between 0 degrees and about 60 degrees (with respect to the surface normal) and for silicon dioxide thicknesses such that the substrate signal was attenuated to not more than 1% and 10% of its original value. Our calculated EALs are in satisfactory agreement with measured EALs., For larger emission angles, the calculated EALs change rapidly with SiO2 thickness, and specific values should be found for the conditions of interest.