The carbides, nitrides, and diborides of the Group IV and Group V transition metal elements are often classified as Ultra-High-Temperature-Ceramics (UHTCs) and are of interest for hypersonic applications. The oxides of these same elements have potential as Environmental Barrier Coatings (EBCs). The mixing behavior in these systems is often poorly understood. As many-component ceramic systems are developed for ever-increasing performance, fundamental thermodynamic data is required to understand and model these materials. This work specifically investigates mixing behavior in the A6B2O17 (A = Zr, Hf | B = Nb, Ta) High-Entropy Oxide system and the (Zr,Ta)B2 UHTC system. In this work, High Temperature Oxide Melt Solution Calorimetry (OMSC) is used to investigate excess enthalpies of mixing. Novel developments are presented for using OMSC with diboride systems. Molar volumes and excess volumes in mixed systems are investigated with Powder X-Ray Diffraction. Results are compared to historical phase diagrams, and implications are discussed.