This thesis details the methodology and findings of a an extensive investigation into the definition and use of thermodynamic properties in the context of the hypersonic flight regime for air species. A surveying of common hypersonic flow solvers is presented, with the majority found to source their properties from the NASA Lewis/Glenn Chemical Equilibrium with Applications (CEA), including the University of Queensland's CFD package Eilmer4. A critical review was then performed on the sources of thermodynamic properties, in an effort to determine whether the current 20, 000K temperature limit of the CEA data could be increased, or more accurately modeled under this temperature. Despite its widespread use, flaws in the CEA modeling approach were identified for both atomic and molecular air species, making it erroneous when compared to the more modern and sophisticated publishings by Mario Capitelli. A complete build of the thermodynamic properties was then implemented using the Capitelli treatment of the internal partition function for both diatomic and monatomic species. Consideration in the monatomic species was made for the choice in ionization potential lowering technique, and a number of extrapolation methods were utilized to complete the electronic spectral levels where experimental observations were found to be lacking. Modeling of diatomic species was performed with consideration for the vibrational-rotational coupling of the atomic nuclei, using a Morse potential approach for determining the maximum permissible vibrational and rotational excitation modes. Quasi stable modes were identified and shown to good agreement with literature results. Good agreement was observed in the resulting thermodynamic properties to modern literature sources, which were then formatted for use in the Eilmer4 project. A preliminary evaluation of the influence these updated properties has on simulated results was presented, however insufficient evidence between the simulated results and available experimental data was found to conclusively validate the improvement made by these new results.