Measurements leading to the calculation of the ideal-gas thermodynamic properties for o-xylene (Chemical Abstracts registry number [95-47-6]) are reported. Experimental methods included adiabatic heat-capacity calorimetry (5 K to 420 K), comparative ebulliometry (313 K to 459 K), differential-scanning calorimetry (DSC), and vibrating-tube densitometry (323 K to 523 K). The critical temperature was measured by DSC. Saturation heat capacities for the liquid phase between 420 K and 550 K, the critical density, and the critical pressure were derived with the vapor-pressure, density, 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 selected 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, Standard entropies are compared with those calculated statistically on the basis of assigned vibrational spectra from the literature for the vapor phase. A preliminary value for the barrier to methyl-group rotation is derived. All results are compared with literature values.