Waveguide coherent anti-Stokes Raman spectroscopy (WCARS) can be used to measure Raman-active vibrations of thin (mono, submono) layers deposited on dielectric waveguides. In these experiments, surface sensitivity and background-free spectra are obtained by using asymmetric mode combinations. The degree of suppression then depends upon the waveguide parameters (thicknesses, refractive indices, and third-order polarizabilities), the wavelengths used, and the selected propagating modes. With knowledge of the refractive indices and the third-order polarizabilities, the other parameters can be used to tune into a background-free region. The refractive indices of many waveguide materials are known or can easily be measured. Unfortunately, the third-order polarizabilities are most often not known and should therefore be determined experimentally. In this paper a method is outlined to measure the third-order susceptibility (X(3)NR) of a waveguide with the use of WCARS experiments. The value X(3)NR of SiOxNy is determined experimentally and is found to be 0.22 ± 0.03 10−12 electrostatic units.