In recent years, high entropy alloys (HEAs) and, more specifically, refractory complex concentrated alloys (RCCAs) have been of increased interest due to their potential as replacements in high temperature environments. This dissertation work has systematically investigated the phase equilibria and oxidation resistance of an alloy system with the basis AlHfNbTiZr. The three alloys investigated in the five-component system were all found to contain a single phase microstructure composed of B2 while the seven component alloy was comprised of a B2 and C14 Laves phase. While the oxidation behavior was parabolic for all alloys studied, the five component alloys exhibited 2-stage parabolic behavior compared to a single stage in the seven-component alloy. The oxidation behavior was governed by a combination of thermodynamics and kinetics with regards to diffusion of oxygen into the system. The five component alloys were found to form an external scale comprised of a ZrTiO4-based structure containing all five elements in their respective stoichiometry. Internal oxidation also occurred in these alloys and exhibited a relationship between diffusion of cations out of the alloy and diffusion of anions into the alloy. However, the seven-component alloy did not form an external scale and was governed completely by oxygen diffusion into the alloy and thermodynamic factors as to the composition of the scale. This work has furthered the fundamental understanding of the oxidation behavior of RCCAs and the accuracy of modelling on the phase equilibria of these alloys.