AbstractAs a first step toward enabling the in-line metrology of III-V nitride heterostructure and materials, we present the optical constants of the two common substrate materials over an unprecendented spectral range. Vacuum Ultraviolet spectroscopic ellipsometry (VUVSE) was used to obtain the optical constants for Al2O3 and the ordinary and extra-ordinary component of the dielectric function for both 4H- and 6H-SiC. The results are discussed in the context of anisotropy, polytypism, bandstructure, optical transitions, and preparation/characterization of abrupt surfaces, where appropriate.