Quantitative determination of oxide nanoparticle composition and structure by X-ray photoelectron spectroscopy (XPS) has proven difficult for metal oxides because of three factors:  some oxide nanoparticles are prone to reduction in the XPS instrument under X-ray illumination in the ultrahigh vacuum (UHV) environment; the nanoparticle structure and integral nature of the XPS technique complicate the data analysis; and the composition is not constant during the finite sampling time required for the XPS experiment. In this report, a method for XPS analysis of core−shell nanoparticle composition and structure is developed to account for these factors quantitatively. The method is applied to characterize the copper(II) oxide (CuO) surface layer on copper(I) oxide (Cu2O) nanoparticles as well as the reduction kinetics of Cu2+ when held under X-ray irradiation in the XPS chamber. The XPS analysis is aided by the availability of copper oxide nanoparticles with a narrow size distribution. When corrected for the finite sampling time, the results show that the reduction reaction follows a second-order rate law, allowing for determination of the true sample composition by extrapolation to zero X-ray exposure time. The initial thicknesses of the CuO surface layer on 6 and 13 nm diameter nanoparticles are estimated to be 0.5 nm by this procedure.