Having been of special interest in thin film technology for a long time already, mixtures of coating materials are still in the focus of research aiming for highest performance in high power as well as in ultra-short pulse laser applications. On the one hand, coating material combinations allow customizing the coating for a certain application by modifying advantageously the refractive index or the band gap energy. On the other hand, that technology is essential in the production of Rugate-filters, using gradually varied refractive index profiles. Therefore, it is of special interest to get insight into the composition of such mixed layers, not only in terms of refractive index and absorption coefficient, but also to evaluate the fractions of materials involved for gaining a better understanding, and therefore to reach highest possible reproducibility for production of such kind of thin films. In this work, single layers of binary mixtures of aluminum oxide, aluminum fluoride, and silicon dioxide are studied with respect to their composition using extreme ultraviolet reflectometry (EUV-R). As the penetration depth of EUV radiation is only a few tens of nanometers under grazing incidence, this non-invasive measurement technique is sensitive to the near surface composition of the film. Therefore it allows investigating the layer material independently of the substrate on which it was deposited. Using specific absorption edges of the involved materials in the EUV spectrum, an empirical correlation between EUV response and mixture ratio is developed and compared to the deep ultraviolet (VUV) absorption edges of the mixture materials. © (2014) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.