We show that a combination of beam profile reflectometry, beam profile ellipsometry, and deep ultraviolet-visible spectroreflectometry can accurately characterize ultrathin oxide on nitride on oxide films with total thicknesses of less than 200 Angstrom. Beam profile ellipsometry establishes the total thickness, while beam profile reflectometry provides the key data to detect the multilayer structure. Optical absorption of the nitride layer at wavelengths shorter than 270 nm as measured by spectroreflectometry provides additional accuracy in determining the thicknesses of the individual layers. The fit to the data is improved by including a '12 Angstrom, interlayer between the Si substrate and the bottom oxide. Determined thicknesses are in excellent agreement with these deduced by cross-sectional transmission electron microscopy.