An ellipsometric method is demonstrated for the assessment of the amount of N in nitrided silicon oxides. The method utilizes the immersion of the film covered sample in a liquid that refractive index matches to the bulk film index. Thus, the overlayer is optically removed yielding greater measurement sensitivity to the interface region where the N is concentrated and is therefore optically distinct. A simple optical model is used and the ellipsometric results are compared with and found in concordance with secondary ion mass spectroscopy N profile measurements made on nitrided oxides that were prepared using N2O and NO. In addition, the interfacial roughness has been found to decrease with increasing N content also in agreement with a literature report. The new method called "spectroscopic immersion ellipsometry" is compared with conventional spectroscopic ellipsometry for the same samples and found to be superior.