Nitrided SiO2 thin films on Si wafers were studied by x-ray reflectivity measurements and their electron-density profiles were evaluated. Interfacial layers of the oxides were found to have densities higher than that of either crystalline Si substrates or strained interfacial layers of thermal oxides. The high density probably results from nitrogen incorporation near the interfaces. The present results suggest that strongly retarded boron penetration through nitrided gate oxides is due to their high-density interfacial layers.