Measurements leading to the calculation of the ideal-gas thermodynamic properties : for ethylbenzene (Chemical Abstracts registry number [100-41-4]) are reported. Experimental methods included adiabatic heat-capacity calorimetry (5 K to 420 K), comparative ebulliometry (306 K to 450 K), and differential-scanning calorimetry(DSC). The critical temperature was measured by DSC. Saturation heat capacities for the Liquid phase between 420 K and 550 K and the critical pressure were derived with the vapor-pressure and DSC results. Results were combined with an enthalpy of combustion reported in the Literature to derive standard molar entropies, enthalpies, and Gibbs free energies of formation at temperatures between 250 K and 550 K. The standard state is defined as the ideal gas at the pressure p = p degrees = 101.325 kPa. Results are compared with literature values for all properties. Problems associated with independent statistical calculation of ideal-gas properties for ethylbenzene are discussed.