The detonation properties of several gaseous systems at elevated initial pressures were examined theoretically. The systems investigated include; hydrogen (H2), methane (CH4), ethylene (C2H4, ethane (C2H6), and propane (C3H8). Chemkin Real Gas, a computer program capable of calculating real-gas thermodynamic properties and chemical kinetic reaction rates, was expanded to utilize the ideal gas, van der Waals, Redlich-Kwong, Soave, and Peng-Robinson equations of state to describe the P-V-T behavior of the gaseous mixtures used in this investigation. The Chapman-Jouguet (CJ) theory of detonation was used to examine the variation of the detonation properties as a function of the initial pressure and temperature. The CJ numerical calculations are shown to be relatively insensitive to equation of state dependent constants (critical properties) estimated for the radical species present in the detonation products. The real-gas equations of state are shown to accurately predict the variation of the experimentally measured detonation velocities with initial pressure.